  Exoplanet atmospheres are the key to understanding the nature of exoplanets.
To this end, transit spectrophotometry provides us opportunities to investigate
the physical properties and chemical compositions of exoplanet atmospheres. We
aim to detect potential atmospheric signatures in 12 gaseous giant exoplanets
using transit spectrophotometry and we try to constrain their atmospheric
properties. The targets of interest were observed using transit
spectrophotometry with the GTC OSIRIS instrument. We estimated the transit
parameters and obtained the optical transmission spectra of the target planets
using a Bayesian framework. We analyzed the spectral features in the
transmission spectra based on atmospheric retrievals. Most of the observed
transmission spectra were found to be featureless, with only the spectrum of
CoRoT-1b showing strong evidence for atmospheric features. However, in
combination with the previously published near-infrared transmission spectrum,
we found multiple interpretations for the atmosphere of CoRoT-1b due to the
lack of decisive evidence for alkali metals or optical absorbers. Featureless
spectra are not necessarily indicative of cloudy atmospheres if they poorly
constrain the altitudes of cloud decks. Precise constraints on the models of
hazes and clouds strongly depend on the significance of the observed spectral
features. Further investigations on these exoplanets, especially CoRoT-1b, are
required to confirm the properties of their atmospheres.
