Orbital period analysis of eclipsing dwarf novae: U Geminorum

A new orbital period analysis for U Geminorum is made by means of the standard O-C technique based on 187 times of light minima including the three newest CCD data from our observation. Although there are large scatter near 70,000 cycles in its O-C diagram, there is strong evidence (>99.9% confidence level) to show the secular increase of orbital period with a rate (P) over dot = 7.8(1) x 10(-12) s(-1). Using the physical parameters recently derived by Echevarr a et al. (Astron. J. 134: 262, 2007), the range of mass transfer rate for U Geminorum is estimated as from-3.5(5) x 10(-9) M-circle dot yr(-1) to-1.30(6) x 10(-8) M-circle dot yr(-1). Moreover, the data before 60,000 cycles shows the obvious quasi-period variations. The least square estimation of a similar to 17.4 yr quasi-periodic variation superimposed on secular orbital period increase is derived. Considering the possibility that solar-type magnetic activity cycles in the secondary star of U Geminorum may produce the quasi-period variations of the orbital period, Applegate's mechanism is discussed and the results indicate such mechanism has difficulty explaining the quasi-period variation for U Geminorum. Hence, we attempted to apply the light-travel time effect to interpret the quasi-period variation and found the perturbation of similar to 17.4 yr quasi-period may result from a brown dwarf. If the orbital inclination is assumed as i similar to 15 degrees, corresponding to the upper limit of mass of a brown dwarf, then its orbital radii is similar to 7.7 AU.