  In order to directly study the role of magnetic fields in the immediate
vicinity of protostars, we use Very-Long-Baseline Interferometry (VLBI), aiming
at the detection of non-thermal centimetric radio emission. This is technically
the only possibility to study coronal emission at sub-AU resolution. We
performed VLBI observations of the four nearby protostars HL Tau, LDN 1551
IRS5, EC 95, and YLW 15 in order to look for compact non-thermal centimetric
radio emission. For maximum sensitivity, we used the High Sensitivity Array
(HSA) where possible, involving the Very Long Baseline Array (VLBA), the phased
Very Large Array (VLA), as well as the Arecibo, Green Bank, and Effelsberg
radio telescopes. While all four protostars were detected in VLA-only data,
only one source (YLW 15 VLA 2) was detected in the VLBI data. The possibility
of non-detections due to free-free absorption, possibly depending on source
geometry, is considered. For YLW 15 VLA 2, the prospects for an accurate orbit
determination appear to be good.
