Difference in magnitude between primary and secondary stars

        :param band:
            Photometric bandpass.

def dmag(self,band):
        """
        Difference in magnitude between primary and secondary stars

        :param band:
            Photometric bandpass.

        """
        mag2 = self.stars['{}_mag_B'.format(band)]
        mag1 = self.stars['{}_mag_A'.format(band)]
        return mag2-mag1