2007A&A...475..309Y

Stellar spots produce periodic variations in photometric data. We aim to determine the observed periods with a high accuracy to improve measurements of stellar differential rotation. We applied the method of three-stage weighted multichannel period analysis (MPA) to photometric observations of LQ Hydrae obtained in 1992-2000. We detected two periods in 10/1993-5/1994, two periods in 10/1998-6/1999, and three periods in 10/1999-3/2000. The primary period was found to be present for all observing seasons with small fluctuations within 1.60-1d.61d, while including the additional secondary periods, the fluctuations become three times larger within 1.585-1d.625d. We also found that the amplitudes of the model curves of the primary period varied and reached their maxima in 10/1993-5/1994 and their minima in 10/1997-6/1998, while the amplitudes of the additional periods were always below the amplitudes of the primary period in the same season. We conclude that the stellar spots responsible for the photometric variations of the primary period are long-lived and change their positions between two latitudes corresponding to two different rotation periods of 1.60d and of 1.61d (or have slightly varying longitude migration rates). The constancy of the primary period in the individual seasons means a constancy of latitude over the observation time intervals. We interpret the appearance of the additional periods to be caused by the formation of new surface structures at different latitudes. The surface structure is thus temporary. Based on our detected period range, the corresponding latitudinal differential rotation rate ΔΩ/Ω=0.025 is small compared with that of the Sun (ΔΩ/Ω=0.2).