2026AJ....171...82S

We present 10 sets of light curves of the contact binary MT Cas, obtained over a span of 12 yr from new multiband photometric observations and archival data from Transiting Exoplanet Survey Satellite and American Association of Variable Star Observers. The light curves display flat-bottomed primary eclipses and a variable O'Connell effect, which we modeled with two starspots-one hot and one cool-on the more massive, cooler primary. Photometric solutions using the Wilson-Devinney code classify MT Cas as a W-subtype contact binary with a mass ratio of q = 2.40, inclination i = 83.7°, temperature difference ΔT = 263 K, fill-out factor f = 14%, and third light contribution of 1% ∼ 6%. A total of 719 times of minimum light over 86 yr, including 651 newly determined epochs, reveals a complex period variation comprising a linear trend, a small secular increase of +3.47 × 10^–9 day yr^–1, and three strictly periodic variations with periods of 2.9247, 8.500, and 18.33 yr. We suggest that the observed period increase results from mass transfer from the secondary to the primary, slightly exceeding angular momentum loss due to magnetic braking. The light-travel time effect periods are found to follow an approximate (1:3:6) commensurability. The dynamical stability and physical implications of the MT Cas system as a multiple system are investigated.