2004MNRAS.352.1041L

New charge-coupled device photometric observations of the W UMa-type binary BX Pegasi (BX Peg) were collected on four nights from 1999 October to 2000 September. The light curve was covered completely in each season. Seven new times of minimum light were determined. It was found that the orbital period of the system has varied recently in a sinusoidal way, superimposed on a downward parabolic variation. The long-term period decrease rate is deduced as dP/dt=-8.62 or 9.59x10^–8 d/yr, which can be interpreted as either mass transfer from the more massive cool star to the less massive hot component, or as the combination of mass transfer and angular momentum loss due to a magnetic stellar wind. The period and amplitude of the sinusoidal period variation were calculated to be about 35.3 yr and 0.015 d, respectively. The light curves of BX Peg are asymmetric and show year-to-year light variability. A spot model has been applied to analyse these light curves. After using the light curves of 1999 as reference ones, we solve those of 2000 by adjusting only the spot parameters. One cool-spot model on the cool secondary satisfies the observed light curves of both 1999 and 2000 quite well and shows a good representation of the BX Peg system for both the photospheric and spot descriptions. The brightness variations of BX Peg are not coincident with the period variations and so do not conform to a prediction of the Applegate mechanism. We think the most likely cause of the cyclical variation is the light-time effect due to a third body, although no third light was detected in the light-curve analysis. If it exists, the hypothetical object could be a very red main-sequence star or a white dwarf. We have solved anew the historical published light curve for only the spot parameters and these closely resemble our spot parameters. We speculate that this result is associated with the small coronal saturation of the cool star of the system.