1999A&A...341..768G

We have investigated the magnetic field strength of one weak-line and four classical T Tauri stars. The magnetic field strength is derived from the differential change of the equivalent width of photospheric FeI lines in the presence of a magnetic field, calculated using a full radiative transfer code. The method was successfully tested by applying it to a non-magnetic solar-type star, and to VY Ari which is believed to have a strong magnetic field. For two of the classical T Tauri stars, we find a product of magnetic field strength and filling factor B.f=(2.35±0.15)kG for T Tau, and B.f=(1.1±0.2)kG for LkCa 15. For the classical T Tauri star UX Tau A and the weak-line T Tauri star LkCa 16 the detection is only marginal, indicating magnetic field strengths of the order of 1kG and possibly of more than 2kG, respectively. No field could be detected for the classical T Tauri star GW Ori. For the two classical T Tauri stars for which we have detected a field, we find the filling factors to be larger than ∼0.5, which indicates that the magnetic field covers most of the photosphere. We also show that ignoring a magnetic field can, depending on the lines used, result in errors in effective temperature and underestimates of veiling.