Abstract
Orbital period variations of the low-mass ratio (q = 0.122) overcontact binary system, GR Vir, were investigated by using two new CCD times of minimum light and other photoelectric data compiled from literatures. It is found that the O - C residuals of GR Vir show a cyclic variation with a period of 19.3 yr and an amplitude of 0.0140 days while they are undergoing a long-term decrease (dP/dt = -4.32 × 10-7 days yr-1). Meanwhile, the 1988 photoelectric observations from Cereda et al. were analyzed using the Wilson-Devinney method. Like some low-mass ratio overcontact binary stars (e.g., AW UMa), GR Vir is an A-type overcontact binary with a high degree of overcontact (f = 78.6%). By combining the spectroscopic solutions with the photometric elements, the absolute parameters of the system are determined as follows: M1 = 1.36 M⊙, M2 = 0.17 M⊙, a = 2.40 R⊙, R1 = 1.42 R⊙, R2 = 0.61 R⊙, L1 = 2.87 L⊙, and L2 = 0.48 L⊙. The long-term period decrease is interpreted as the result of mass transfer from the more massive component to the less massive one in combination with the angular momentum loss due to mass outflow from the L2 point. The conditions in GR Vir resemble those in AW UMa. Both systems show a high degree of overcontact, low mass ratios, and secular shrinking of their orbits. As their orbital periods decrease, the shrinking of the inner and outer critical Roche lobes will cause the common convective envelope to become deeper, until finally the formation of single, rapid-rotation stars is inevitable. The period oscillation may by caused either by the presence of an unseen tertiary component (e.g., a white dwarf) or by magnetic activity on the part of the primary component.