3_1  One of the two optical counterparts of this source has been identified as
3_1   a variable star. More details of source 3_1 are presented in Section A.4.
3_4  A single optical match was identified in both OGLE-II and MCPS catalogs.
3_4   However, its V=18.19 magnitude and B-V=0.37 color are not indicative of
3_4   the nature of the companion star. Its soft X-ray spectrum (A. Zezas et al.
3_4   2009, in preparation) possibly suggests an AGN.
5_9  Chandra sources 5_9 and 6_8 have possibly a soft X-ray spectrum and
5_9   multiple optical matches within their search radius. The brightest matches
5_9   of 5_9 and 6_8 are not O or B-type stars (B-V color~0.59 and ~1.12mag,
5_9   respectively). Recently, Laycock et al. (2010ApJ...716.1217L) detected
5_9   source 5_9 as a candidate quiescent X-ray pulsar (SXP7.83).
6_8  Chandra sources 5_9 and 6_8 have possibly a soft X-ray spectrum and
6_8   multiple optical matches within their search radius. The brightest matches
6_8   of 5_9 and 6_8 are not O or B-type stars (B-V color~0.59 and ~1.12mag,
6_8   respectively). 
5_15 Within the search radius of this source we identified five matches, all of
5_15  them fainter than V~19mag. Its possibly soft X-ray spectrum suggests an
5_15  AGN. However, Laycock et al. (2010ApJ...716.1217L) detected this source as
5_15  a candidate quiescent X-ray pulsar, while the brightest of its matches has
5_15  been identified as a Be candidate star (Mennickent et al. 
5_15  2002A&A...393..887M). See also the discussion in Section 9.
5_23 We identify MCPS source Z_2288276 (no OGLE-II overlap) as the optical
5_23  counterpart of X-ray source 5_23. Based on its V magnitude and B-V color,
5_23  this is an O or B-type star. Thus, we classify this source as a new
5_23  candidate HMXB. The small number of net counts of source 5_23 (~10 counts
5_23  in the 0.5-7.0keV energy band; A. Zezas et al. 2009, in preparation) does
5_23  not allow us to derive its X-ray spectral information, thus prohibiting us
5_23  to classify it further as a new candidate Be-XRB.
4_10 Chandra sources 4_10, 5_6, 5_32, 6_3, 7_17: The soft X-ray spectrum of
4_10  these sources (HR method, more details in Table 7 and its description in
4_10  Section 8.3) and their early-type counterparts could be indicative of a
4_10  candidate black hole X-ray binary (BH-XRB) nature. However, only X-ray
4_10  spectral fits and optical spectroscopy of the companion stars could
4_10  possibly identify their true nature.
5_6  Chandra sources 4_10, 5_6, 5_32, 6_3, 7_17: The soft X-ray spectrum of
5_6   these sources (HR method, more details in Table 7 and its description in
5_6   Section 8.3) and their early-type counterparts could be indicative of a
5_6   candidate black hole X-ray binary (BH-XRB) nature. However, only X-ray
5_6   spectral fits and optical spectroscopy of the companion stars could
5_6   possibly identify their true nature.
5_32 Chandra sources 4_10, 5_6, 5_32, 6_3, 7_17: The soft X-ray spectrum of
5_32  these sources (HR method, more details in Table 7 and its description in
5_32  Section 8.3) and their early-type counterparts could be indicative of a
5_32  candidate black hole X-ray binary (BH-XRB) nature. However, only X-ray
5_32  spectral fits and optical spectroscopy of the companion stars could
5_32  possibly identify their true nature.
6_3  Chandra sources 4_10, 5_6, 5_32, 6_3, 7_17: The soft X-ray spectrum of
6_3   these sources (HR method, more details in Table 7 and its description in
6_3   Section 8.3) and their early-type counterparts could be indicative of a
6_3   candidate black hole X-ray binary (BH-XRB) nature. However, only X-ray
6_3   spectral fits and optical spectroscopy of the companion stars could
6_3   possibly identify their true nature.
7_17 Chandra sources 4_10, 5_6, 5_32, 6_3, 7_17: The soft X-ray spectrum of
7_17  these sources (HR method, more details in Table 7 and its description in
7_17  Section 8.3) and their early-type counterparts could be indicative of a
7_17  candidate black hole X-ray binary (BH-XRB) nature. However, only X-ray
7_17  spectral fits and optical spectroscopy of the companion stars could
7_17  possibly identify their true nature.
6_20 The small number of net counts of this source (less than 10 counts in the
6_20  0.5-7.0keV energy band; A. Zezas et al. 2009, in preparation) does not
6_20  allow us to derive spectral information for it. Its V=13.71 magnitude and
6_20  B-V=0.39 color are not indicative of the nature of the companion star,
6_20  thus this source remains unclassified in this work. More details on this
6_20  source are presented in Section 3.
7_17 This source has possibly a soft X-ray spectrum. The large search radius of
7_17  it (5.29", maximum search radius used in this study because of the large
7_17  off-axis angle) resulted in the identification of multiple matches.
7_17  However, only two sources are bright (V~18.4 mag, thus resulting in
7_17  smaller chance coincidence probability). One of the two sources lies on
7_17  the red giant branch, while the other one (O_5_140992) falls just outside
7_17  the locus of OB spectral type stars. Source 7_17 remains unclassified in
7_17  this work.
7_19 Due to the small number of counts (less than 10 counts in the 0.5-7.0keV
7_19  energy band; A. Zezas et al. 2009, in preparation) we do not have X-ray
7_19  spectral information for this source. However, there is an emission-line
7_19  object within ~2{sigma} of the search radius (source [MA93]316;
7_19  Meyssonnier & Azzopardi 1993, Cat. J/A+AS/102/451). Actually this source
7_19  is located in the star cluster SMC-N32 (Bica & Schmitt 1995, 
7_19  Cat. J/ApJS/101/41), a crowded region surrounded by a diffuse emission
7_19  (HII region). The detection algorithm in this case resulted in many
7_19  spurious sources that they were not deblended (see also the online finding
7_19  chart for this source). We note here that the brightest of the five
7_19  matches within the search radius of this source is located just to the
7_19  right border of the ellipse of OB stars in the V, B-V CMD. Its large error
7_19  in B-V color could place it within the ellipse, thus suggesting that it
7_19  could be another new HMXB.
4_32 Chandra sources 4_32, 4_36, 5_18, 5_25, 6_23, 7_9: All but two of the
4_32  brightest counterparts of these sources lie on the red giant branch in the
4_32  V, B-V CMD (Figure 3), while that of source 5_18 lies on the right of the
4_32  MS (most probably in the giant branch). The counterpart of source 7_9 has
4_32  M_V_0__=-5.7 and (B-V)_0_>0.75, thus it lies on the locus of foreground
4_32  stars (for more details see Section 6). In addition, these sources do not
4_32  have X-ray spectral information (because of their small number of counts),
4_32  except from source 7_9 which has possibly a hard X-ray spectrum.
4_36 Chandra sources 4_32, 4_36, 5_18, 5_25, 6_23, 7_9: All but two of the
4_36  brightest counterparts of these sources lie on the red giant branch in the
4_36  V, B-V CMD (Figure 3), while that of source 5_18 lies on the right of the
4_36  MS (most probably in the giant branch). The counterpart of source 7_9 has
4_36  M_V_0__=-5.7 and (B-V)_0_>0.75, thus it lies on the locus of foreground
4_36  stars (for more details see Section 6). In addition, these sources do not
4_36  have X-ray spectral information (because of their small number of counts),
4_36  except from source 7_9 which has possibly a hard X-ray spectrum.
5_18 Chandra sources 4_32, 4_36, 5_18, 5_25, 6_23, 7_9: All but two of the
5_18  brightest counterparts of these sources lie on the red giant branch in the
5_18  V, B-V CMD (Figure 3), while that of source 5_18 lies on the right of the
5_18  MS (most probably in the giant branch). The counterpart of source 7_9 has
5_18  M_V_0__=-5.7 and (B-V)_0_>0.75, thus it lies on the locus of foreground
5_18  stars (for more details see Section 6). In addition, these sources do not
5_18  have X-ray spectral information (because of their small number of counts),
5_18  except from source 7_9 which has possibly a hard X-ray spectrum.
5_25 Chandra sources 4_32, 4_36, 5_18, 5_25, 6_23, 7_9: All but two of the
5_25  brightest counterparts of these sources lie on the red giant branch in the
5_25  V, B-V CMD (Figure 3), while that of source 5_18 lies on the right of the
5_25  MS (most probably in the giant branch). The counterpart of source 7_9 has
5_25  M_V_0__=-5.7 and (B-V)_0_>0.75, thus it lies on the locus of foreground
5_25  stars (for more details see Section 6). In addition, these sources do not
5_25  have X-ray spectral information (because of their small number of counts),
5_25  except from source 7_9 which has possibly a hard X-ray spectrum.
6_23 Chandra sources 4_32, 4_36, 5_18, 5_25, 6_23, 7_9: All but two of the
6_23  brightest counterparts of these sources lie on the red giant branch in the
6_23  V, B-V CMD (Figure 3), while that of source 5_18 lies on the right of the
6_23  MS (most probably in the giant branch). The counterpart of source 7_9 has
6_23  M_V_0__=-5.7 and (B-V)_0_>0.75, thus it lies on the locus of foreground
6_23  stars (for more details see Section 6). In addition, these sources do not
6_23  have X-ray spectral information (because of their small number of counts),
6_23  except from source 7_9 which has possibly a hard X-ray spectrum.
7_9  Chandra sources 4_32, 4_36, 5_18, 5_25, 6_23, 7_9: All but two of the
7_9   brightest counterparts of these sources lie on the red giant branch in the
7_9   V, B-V CMD (Figure 3), while that of source 5_18 lies on the right of the
7_9   MS (most probably in the giant branch). The counterpart of source 7_9 has
7_9   M_V_0__=-5.7 and (B-V)_0_>0.75, thus it lies on the locus of foreground
7_9   stars (for more details see Section 6). In addition, these sources do not
7_9   have X-ray spectral information (because of their small number of counts),
7_9   except from source 7_9 which has possibly a hard X-ray spectrum.
4_16 This is a point source associated with the previously known SNR 0047-735
4_16  (Mathewson et al. 1984ApJS...55..189M). For this source, one single
4_16  optical match (O_5_27768) has been found, while it has been recently
4_16  classified as a new candidate quiescent X-ray pulsar (CXO J004905.1-731411
4_16  =SXP8.94; Laycock et al. 2010ApJ...716.1217L).
4_20 This point source is associated with the previously known SNR 0049-73.6
4_20  (Mathewson et al. 1984ApJS...55..189M). We identify source O_5_262679 as
4_20  the optical counterpart of source 4_2. CXO J005059.0-732055 (SXP9.39;
4_20  Laycock et al. 2010ApJ...716.1217L) has been classified as a candidate
4_20  quiescent X-ray pulsar.
4_21 This point source is also associated with the SNR 0049-73.6 (Mathewson et
4_21  al. 1984ApJS...55..189M; Sasaki et al. 2000, Cat. J/A+AS/147/75). There
4_21  are three bright counterparts (M_V_0__<-0.25) within its search radius,
4_21  while one of those (O_5_256826) is an early-type star (O or B).
3_3  The identified counterpart O_8_49531 (single match) of X-ray source 3_3 is
3_3   a Be candidate star ([MPG02]No.494), in agreement with the identification
3_3   of Sasaki et al. (2003A&A...403..901S).
4_2  We find two matches for X-ray source 4_2. Although none of them are located
4_2   within the locus of OB stars (criterion (1)), we identify source
4_2   O_5_111490 as the counterpart for 4_2, because of its Be nature
4_2   ([MPG02]No.206), while it was also found to be a variable star (Ita et al.
4_2   2004, Cat. J/MNRAS/353/705). This is in agreement with the identification
4_2   of Coe et al. (2005MNRAS.356..502C).
4_17 Source O_5_180008 is identified as the optical counterpart of X-ray source
4_17  4_17, while it is also identified as a Be candidate star ([MPG02]No.265).
4_17  This source coincides with the emission-line object [MA93]396. In
4_17  combination with the possibly hard X-ray spectrum, we classify this source
4_17  as a new Be-XRB.
5_3  The identified optical counterpart O_6_85614 (in agreement with Schmidtke &
5_3   Cowley 2006AJ....132..919S) is a Be candidate star [MPG02]No.321. It is of
5_3   B0-5 spectral type (source [2dF]No.0828; see Section 3), and it has been
5_3   found to coincide with the emission-line object [MA93]No.506 (in agreement
5_3   with Haberl & Sasaki 2000A&A...359..573H).
5_7  The identified optical counterpart of X-ray source 5_7 is source O_7_70829.
5_7   It is identified as a Be candidate star ([MPG02]No.426), while Sasaki et
5_7   al. (2003A&A...403..901S) presented the same counterpart, identified also
5_7   as the emission-line object [MA93]No.810.
5_15 We identify source O_7_71429 as the optical counterpart of X-ray source
5_15  5_15 (see also Section 9). This is also reported as a Be candidate star
5_15  ([MPG02]No.423). However, Dobrzycki et al. (2003AJ....125.1330D)
5_15  identified this object as a quasar (z=1.79) located behind the SMC (based
5_15  on a spectroscopic study).
6_1  The identified counterpart O_6_77228 of source 6_1 is a candidate Be star
6_1   ([MPG02]No.341; see also Section 3).
6_4  Source O_6_147662 is identified as the optical counterpart of X-ray source
6_4   6_4. This optical source is also reported as a candidate Be star
6_4   ([MPG02]No.364), and it is located ~0.57" away from the emission-line
6_4   object [MA93]618 (in agreement with Haberl & Sasaki 2000A&A...359..573H).
7_1  We identify source O_5_65517 as the optical counterpart of X-ray source
7_1   7_1. This is star 1 of Stevens et al. (1999MNRAS.309..421S), and it is
7_1   also in agreement with the identification of Coe & Orosz
7_1   (2000MNRAS.311..169C). This OGLE source is identified as a Be candidate
7_1   star ([MPG02]No.199).
3_1  We identify source O_7_267163 as the optical counterpart of X-ray source
3_1   3_1. From visual inspection of the finding chart, from comparison of the
3_1   V magnitude for sources O_7_267163 and O_7_267132, and taking into account
3_1   the ~0.7" astrometric accuracy of OGLE, we believe that these stars are a
3_1   single source. Although none of the identified matches are an early-type
3_1   star, and the parameter {xi} cannot distinguish in this case the right
3_1   counterpart, source O_7_267163 is identified as a variable star
3_1   (OGLE00571981-72253375) in the study of Zebrun et al. (2001, 
3_1   Cat. J/AcA/51/317). Sasaki et al. (2003A&A...403..901S) identified the
3_1   same object, while they also associated it with star 
3_1   USNOA2.0 0150-00625436. The soft X-ray spectrum of 3_1 and the
3_1   identification of the optical counterpart as a background quasar
3_1   (Dobrzycki et al. 2003AJ....126..734D) suggest that this is possibly an
3_1   AGN (in agreement with Sasaki et al. 2003A&A...403..901S).
4_2  Source O_5_111490 is also identified as a variable star in the analysis of
4_2   Ita et al. (2004, Cat. J/MNRAS/353/705; see also Section 9).
4_5  The optical counterpart of X-ray source 4_5 is the variable star O_5_111500
4_5   (single optical match; P_pulse,opt_=89.835days; Ita et al. 2004, 
4_5   Cat. J/MNRAS/353/705). Haberl et al. (2008A&A...489..327H) have recently
4_5   confirmed the Be-XRB nature of this source, while no significant
4_5   pulsations have been found in their study. In a later work, Laycock et al.
4_5   (2010ApJ...716.1217L) identified Chandra source 4_5 as the X-ray pulsar
4_5   SXP892.
4_11 Source O_4_164855 is the single match of Chandra source 4_11 within the
4_11  search radius with V magnitude and B-V color which place it just below the
4_11  locus of OB stars in the CMD. We mention here that in ~2{sigma} from the
4_11  X-ray source position there is a bright variable star (O_4_163513=
4_11  OGLE 004802.55-731659.8; Ita et al. 2004, Cat. J/MNRAS/353/705). However,
4_11  the B-V color of this optical source is not indicative of an O or B-type
4_11  star, thus even if its X-ray spectrum is possibly hard this source remains
4_11  unclassified.
5_1  The identified optical counterpart in this case is MCPS source Z_2311496,
5_1   in agreement with Coe et al. (2005MNRAS.356..502C). This source is also
5_1   defined as a B0-5(II)e star in the 2dF spectroscopic catalog of Evans et
5_1   al. (2004, Cat. J/MNRAS/353/601; source [2dF]No.0839).
5_3  The optical counterpart O_6_85614 of source 5_3 is identified as a B0-5(II)
5_3   spectral-type source ([2dF]No.0828).
5_16 A single match (source Z_2573354) is identified as the optical counterpart
5_16  of this source in the MCPS catalog (no OGLE-II overlap), in agreement with
5_16  Coe et al. (2005MNRAS.356..502C). Buckley et al. (2001MNRAS.320..281B)
5_16  identified source 5_16 as a transient pulsar and spectroscopically
5_16  classified the suggested counterpart (star A or B mentioned in their
5_16  study) as a B1-B2III-Ve star. On the other hand, the variable MACHO source
5_16  207.16202.30 lies >14" away of X-ray source 5_16 (unlikely the correct
5_16  counterpart; Coe et al. 2005MNRAS.356..502C).
6_1  We identify source O_6_77228 as the optical counterpart of X-ray source
6_1   6_1 (in agreement with Coe et al. 2005MNRAS.356..502C). This is source
6_1   [2dF]No.5054 classified as a B1-5(II)e star (Evans et al. 2004,
6_1   Cat. J/MNRAS/353/601). It also appears as a candidate Be star in the
6_1   compilation of Mennickent et al. (2002A&A...393..887M; [MPG02]No.341). For
6_1   more details see Section 9.
6_20 Source O_6_311169 (single match) is proposed here as the optical
6_20  counterpart of X-ray source 6_20. It is the brightest counterpart
6_20  identified in this study, while it has been identified as the
6_20  emission-line object [MA93]No.739 (peculiar H{alpha} emission-line star
6_20  with FeII and [FeII] emission; ~0.17" from the OGLE-II source, and ~0.8"
6_20  from the X-ray source). The latter source has been identified as a B[e]
6_20  star by Massey & Duffy (2001ApJ...550..713M). We only propose the above
6_20  OGLE-II source as the counterpart of Chandra source 6_20, and we note that
6_20  further analysis of this peculiar system is required so as to confirm its
6_20  nature (see also Appendix 9).
3_2  This source is a pulsar (Edge et al. 2004MNRAS.353.1286E). However, both of
3_2   its optical matches in the MCPS catalog presented in Table 2 (no overlap
3_2   with OGLE-II) are faint stars. Due to the high chance coincidence
3_2   probability of faint sources, we do not propose any of them as counterpart
3_2   of X-ray source 3_2. Instead, looking for counterparts between the
3_2   1{sigma} and 2{sigma} search radii, we find MCPS source Z_2806702 (in
3_2   agreement with Edge et al. 2004MNRAS.353.1286E). This is a bright star
3_2   (V=16.78, B-V =-0.12), which is located 4.27" away from the X-ray
3_2   position. Although no spectroscopic data for these three sources are
3_2   available, the early-type nature of MCPS source Z_2806702 makes us believe
3_2   that this is the most likely counterpart. Recently, Haberl & Eger
3_2   (2008ATel.1529....1H) suggested that Chandra source 3_2 is a background
3_2   AGN and not a Be-XRB pulsar, with an optical counterpart one of the two
3_2   faint stars with V=20.3mag found in the 1{sigma} search radius.
4_13 We have identified source O_5_90858 as the optical counterpart of 4_13.
4_13  However, when we look for counterparts between the 1{sigma} and 2{sigma}
4_13  search radii, we find two sources brighter than O_5_90858. These sources,
4_13  O_5_90493 and O_5_90535 (5.47" and 4.10" away from the X-ray source,
4_13  respectively), are both of OB spectral type (V=15.57, B-V=-0.07 and
4_13  V=16.34, B-V=0.23, respectively). The possibly hard X-ray spectrum of 4_13
4_13  suggests that this is a new candidate Be-XRB. However, only optical
4_13  spectroscopy could determine the exact type of these bright stars and the
4_13  true counterpart of Chandra source 4_13.
4_19 The optical source O_5_261182 is found 2.27" away from Chandra source 4_19
4_19  (between the 1{sigma} and 2{sigma} search radius), and it is an OB
4_19  spectral-type star (V=18.16, B-V=-0.08). In combination with its possibly
4_19  hard X-ray spectrum, we propose this as a new HMXB. Optical spectroscopy
4_19  will definitely determine if this source is also a new Be-XRB.