  We observed the Saturn-mass and Jupiter-sized exoplanet HAT-P-19b to refine
its transit parameters and ephemeris as well as to shed first light on its
transmission spectrum. We monitored the host star over one year to quantify its
flux variability and to correct the transmission spectrum for a slope caused by
starspots. A transit of HAT-P-19b was observed spectroscopically with OSIRIS at
the Gran Telescopio Canarias in January 2012. The spectra of the target and the
comparison star covered the wavelength range from 5600 to 7600 AA. One
high-precision differential light curve was created by integrating the entire
spectral flux. This white-light curve was used to derive absolute transit
parameters. Furthermore, a set of light curves over wavelength was formed by a
flux integration in 41 wavelength channels of 50 AA width. We analyzed these
spectral light curves for chromatic variations of transit depth. The transit
fit of the combined white-light curve yields a refined value of the
planet-to-star radius ratio of 0.1390 pm 0.0012 and an inclination of 88.89 pm
0.32 degrees. After a re-analysis of published data, we refine the orbital
period to 4.0087844 pm 0.0000015 days. We obtain a flat transmission spectrum
without significant additional absorption at any wavelength or any slope.
However, our accuracy is not sufficient to significantly rule out the presence
of a pressure-broadened sodium feature. Our photometric monitoring campaign
allowed for an estimate of the stellar rotation period of 35.5 pm 2.5 days and
an improved age estimate of 5.5^+1.8_-1.3 Gyr by gyrochronology.
