A cool starspot or a second transiting planet in the TrES-1 system?

Aims. We investigate the origin of a flux increase found during a transit of TrES-1, observed with the HST (Hubble Space Telescope). This feature in the HST light curve cannot be attributed to noise and is supposedly a dark area on the stellar surface of the host star eclipsed by TrES-1 during its transit. We investigate the likelihood of two possible hypotheses for its origin. A starspot or a second transiting planet. Methods. We made use of several transit observations of TrES-1 from space with the HST and from ground with the IAC 80-cm telescope (IAC-80). On the basis of these observations we did a statistical study of flux variations in each of the observed events to investigate whether similar flux increases are present in other parts of the data set. Results. The HST observation presents a single clear flux rise during a transit, whereas the ground observations lead to detecting two such events but with low significance. In the case of having observed a starspot in the HST data, assuming a central impact between the spot and TrES-1, we would obtain a lower limit for the spot radius of 42 000 km. For this radius the spot temperature would be 4690 K, 560 K lower then the stellar surface of 5250 K. For a putative second transiting planet, we can set a lower limit for its radius at 0.37 R_J and for periods of less than 10.5 days, we can set an upper limit at 0.72 R_J. Conclusions. Assuming a conventional interpretation, this HST observation then constitutes the detection of a starspot. Alternatively, this flux rise might also be caused by an additional transiting planet. The true nature of the origin can be revealed if a wavelength dependency of the flux rise can be shown or discarded with higher certainty. Additionally, the presence of a second planet can be detected by radial velocity measurements.