Context. The CoRoT mission was the first dedicated to the search for exoplanets from space. The CoRoT exoplanet channel observed about 163 600 targets to detect transiting planetary companions. In addition to the search for exoplanets, the extremely precise photometric time series provided by CoRoT for this vast number of stars is an invaluable resource for stellar studies. Because CoRoT targets are faint (11 ≤ r ≤ 16) and close to the galactic plane, only a small subsample has been observed spectroscopically. Consequently, the stellar classification of CoRoT targets required the design of a classification method suited for the needs and time frame of the mission. Aims. We describe the latest classification scheme used to derive the spectral type of CoRoT targets, which is based on broadband multi-colour photometry. We assess the accuracy of this spectral classification for the first time. Methods. We validated the method on simulated data. This allows the quantification of the effect of different sources of uncertainty on the spectral type. Using galaxy population synthesis models, we produced a synthetic catalogue that has the same properties as the CoRoT targets. In this way, we are able to predict typical errors depending on the estimated luminosity class and spectral type. We also compared our results with independent estimates of the spectral type. Cross-checking those results allows us to identify the systematics of the method and to characterise the stellar populations observed by CoRoT. Results. We find that the classification method performs better for stars that were observed during the mission-dedicated photometric ground-based campaigns.The luminosity class is wrong for less than 7% of the targets. Generally, the effective temperature of stars classified as early type (O, B, and A) is overestimated. Conversely, the temperature of stars classified as later type tends to be underestimated. This is mainly due to the adverse effect of interstellar reddening. We find that the median error on the effective temperature is less than 5% for dwarf stars classified with a spectral later than F0, but it is worse for earlier type stars, with up to 20% error for A and late-B dwarfs, and up to 70% for early-B and O-type dwarfs. Similar results are found for giants, with a median error that is lower than 7% for G-and later type giants, but greater than 25% for earlier types. Overall, we find an average median absolute temperature difference |∆T eff | = 533 ± 6 K for the whole sample of stars classified as dwarfs and |∆T eff | = 280 ± 3 K for the whole sample of giant stars. The corresponding standard deviation is of about 925 ± 5 K for dwarfs and 304 ± 4 K for giants. Typically for late-type stars, this means that the classification is accurate to about half a class.

We present newly reduced archival radio observations of SN 1996cr in the Circinus Galaxy from the Australia Telescope Compact Array (ATCA) and the Molonglo Observatory Synthesis Telescope (MOST), and attempt to model its radio light curves using recent hydrodynamical simulations of the interaction between the SN ejecta and the circumstellar material (CSM) at X-ray wavelengths. The radio data within the first 1000 days show clear signs of free-free absorption (FFA), which decreases gradually and is minimal above 1.4 GHz after day ∼3000. Constraints on the FFA optical depth provide estimates of the CSM free electron density, which allows insight into the ionisation of SN 1996cr's CSM and offers a test on the density distribution adopted by the hydrodynamical simulation. The intrinsic spectral index of the radiation shows evidence for spectral flattening, which is characterised by α = 0.852 ± 0.002 at day 3000 and a decay rate of ∆α = -0.014 ± 0.001 yr −1 . The striking similarity in the spectral flattening of SN 1987A, SN 1993J, and SN 1996cr suggests this may be a relatively common feature of SNe/CSM shocks. We adopt this spectral index variation to model the synchrotron radio emission of the shock, and consider several scalings that relate the parameters of the hydrodynamical simulation to the magnetic field and electron distribution. The simulated light curves match the large-scale features of the observed light curves, but fail to match certain tightly constraining sections. This suggests that simple energy density scalings may not be able to account for the complexities of the true physical processes at work, or alternatively, that the parameters of the simulation require modification in order to accurately represent the surroundings of SN 1996cr.