RS Oph spectral type. The spectral type of RS Oph has been measured many times (see Table 24). There is no correlation with orbital phase, likely because RS Oph is being viewed not far from pole-on. But there is a highly significant correlation between the visual magnitude and the spectral type. When RS Oph is faint the spectral class is M4, while when RS Oph is bright the spectral class in K4, with roughly linear variations between. This phenomenon is easy to understand, as a bright RS Oph implies a more luminous white dwarf and accretion disk which translates into a greater heating (and earlier spectral class) for the hemisphere facing the white dwarf. In this plot, I quantified the spectral class as K3 going to −3, K4 going to −2, K5 going to −1, M0 going to 0, M1 going to 1, and so on up to M5 going to 5.