  We set to search for Rayleigh scattering and K and Na absorption signatures
from the atmosphere of TrES-3b using ground-based transmission spectroscopy
covering the wavelength range from 530 to 950 nm as observed with OSIRIS@GTC.
  Our analysis is based on a Bayesian approach where the light curves covering
a set of given passbands are fitted jointly with PHOENIX-calculated stellar
limb darkening profiles. The analysis is carried out assuming both white and
red -- temporally correlated -- noise, with two approaches (Gaussian processes
and divide-by-white) to account for the red noise.
  An initial analysis reveals a transmission spectrum that shows a strong
Rayleigh-like increase in extinction towards the blue end of the spectrum, and
enhanced extinction around the K I resonance doublet near 767 nm. However, the
signal amplitudes are significantly larger than expected from theoretical
considerations. A detailed analysis reveals that the K I-like feature is
entirely due to variability in the telluric O$_2$ absorption, but the
Rayleigh-like feature remains unexplained.
