Plausible explanations for the variations of orbital period in the old nova DQ Herculis

Abstract

Aims. Several mechanisms are presented to explain the observed small variation in the orbital period of the old nova DQ Herculis. Methods. We have combined two new CCD times of light minimum of DQ Herculis with all 226 available times of light minimum, including 79 visual observations, for the new orbital period analysis. Results. Based on this analysis, the best-fit of the O-C diagram for DQ Herculis is a quadratic-plus- sinusoidal fit. A secular orbital period increase with a rate of 9.5(+/- 0.1) x 10(-12) s s(-1) is confirmed, which corresponds to a lower limit of the mass transfer rate of 7.2(+/- 3.2) x 10(-9) M(circle dot) yr(-1). We investigate three plausible mechanisms (direct change of the red dwarf's radius, Applegate's mechanism and the light travel-time effect) to explain the quasi-periodic variation shown in the O-C diagram. Although previous works have suggested that solar-type magnetic cycles in the red dwarf can explain the quasi-periodic variation in the orbital period, we were not able to reproduce this finding. Accordingly, a light trave-time effect is proposed, with a brown dwarf as a tertiary component with a significance level of greater than or similar to 77.8% orbiting around nova DQ Herculis. In order to interpret the small departure from the best-fit near 60 000 cycles, we assume an eccentric orbit of the third body with a small eccentricity. However, a satisfying result was obtained because the eccentricity e = 0.12 is close to zero. The parameters of this elliptical orbit are similar to that of a circular orbit.