Rotational period of GQ Lupi

Aims. We wanted to determine the rotation parameters of GQ Lup A, thereby constraining the evolutionary history of the GQ Lup system. Methods. We have undertaken a photometric monitoring campaign on GQ Lup A consisting of two epochs spaced one year apart. We also searched the photometric archives to enlarge the data set. Results. We were able to determine the photometric period (8.45 +/- 0.2 days) in both epochs in several photometric bands. This periodicity could also be found in some of the archival data. The combined false-alarm probability is 0.015. The variation is most likely caused by hot spots on the surface of GQ Lup A. This, combined with high-resolution spectra (upsilon sin i) allows calculation of GQ Lup A's inclination (i = 27 +/- 5 degrees). Radial velocity data also contains this period but is inconclusive. Nevertheless, the RV data supports the interpretation that hot spots cause the photometric variation. We use the known K-band variability, amplitude, and phase of GQ Lup A together with a new image of GQ Lup A+b, taken quasi-simultaneously with our monitoring of the star, to confirm the magnitude and, hence, luminosity of the companion.