Fig. 4. Line-depth ratio variations of IN Virginis (upper panels) and their respective calibrations from luminosity-class III and IV M-K standard stars against observed color (lower panels). The dots are the measured line ratios and their error bars indicate the whole range of values from repeated measurements with different techniques. The dotted line in the upper panels is the scaled, simultaneous V -band light curve and emphasizes the relation with the line-ratio variations due to the common cause. Note that the line ratios are chosen with temperature-sensitive line over temperature-insensitive line, therefore, the larger the line ratio the stronger was the temperature-sensitive line, and thus the cooler the average surface temperature. The crosses in the lower panels are the standard-star observations and the lines are the fits with the second-order polynomials in Eq. (2).