Measurements of magnetic field strength on T Tauri stars

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 di fferential change of the equivalent width of photospheric Fe I lines in the presence of a magnetic field, calculated using a full radiative transfer c ode. The method was successfully tested by applying it to a nonmagnetic sol ar-type star, and to VY Ari which is believed to have a strong magnetic fie ld. For two of the classical T Tauri stars, we find a product of magnetic f ield 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, i ndicating magnetic field strengths of the order of 1 kG and possibly of mor e than 2 kc, respectively. No field could be detected for the classical T T auri star GW Ori. For the two classical T Tauri stars for which we have det ected a field, we find the filling factors to be larger than similar to 0.5 , which indicates that the magnetic field covers most of the photosphere. W e also show that ignoring a magnetic field can, depending on the lines used , result in errors in effective temperature and underestimates of veiling.