2008A&A...489..349S

GQ Lup A is a classical T Tauri star that shows clear signs of accretion through the presence of inverse P Cygni profiles in its main emission lines. Recently, Neuhaeuser et al. (2005A&A...435L..13N) found a co-moving sub-stellar companion of GQ Lup A, raising the importance of determining precise stellar parameters, system age, and distance to GQ Lup. Our main objective is to use spectrophotometric time series to determine GQ Lup A stellar parameters and predict its photospheric spectral distribution. The excess spectral luminosity can then be measured and employed to test predictions of magnetospheric accretion models of classical T Tauri stars. We present the analysis of 18 spectrophotometric observations of the T Tauri star GQ Lup A (K7) obtained with the Boller & Chivens spectrograph at the 1.52m ESO telescope in La Silla. We also revisited archival photometric data of this star, comparing previous light curve variability with our more recent data. We determined the photospheric flux of GQ Lup A on each observing night and obtained the stellar radius (R_*=1.8±0.3R_☉), adopting a mean distance of 150±20pc to the Lupus 1 cloud. Assuming a K7 V temperature of 4060K, the luminosity of GQ Lup A is L_*=0.8±0.3L_☉. Standard evolutionary models indicate a stellar mass of M_*=0.8±0.2M_☉ and an age of 3±2Myr. GQ Lup A spectral lines are consistent with a projected rotational velocity of vsini=6.5±2.0km/s. We measured the excess emission - veiling - and used the resulting photospheric spectral distribution to calculate a stellar extinction (A_V) of 0.5±0.1. The veiling was found to be variable and periodic at 10.7±1.6days, which is consistent with the period of GQ Lup A obtained from archival B band photometric data (10.43±0.12days). The star exhibits strong emission lines with substantial variability in flux. The emission line fluxes are strongly correlated with one another but not with veiling.