  Direct measurements of the stellar magnetic fields are based on the splitting
of spectral lines into polarized Zeeman components. With few exceptions, Zeeman
signatures are hidden in data noise and a number of methods have been developed
to measure the average, over the visible stellar disk, of longitudinal
components of the magnetic field. As to faint stars, at present observable only
with low resolution spectropolarimetry, a method is based on the regression of
the Stokes V signal against the first derivative of Stokes I. Here we present
an extension of this method to obtain a direct measurement of the transverse
component of stellar magnetic fields by the regression of high resolution
Stokes Q and U as a function of the second derivative of Stokes I. We also show
that it is possible to determine the orientation in the sky of the rotation
axis of a star on the basis of the periodic variability of the transverse
component due to its rotation. The method is applied to data, obtained with the
Catania Astrophysical Observatory Spectropolarimeter, along the rotational
period of the well known magnetic star \b{eta} CrB.
