  We present longitudinal magnetic field measurements $\langle B_z \rangle$ for
the Herbig Ae primary and the T Tauri secondary in the SB2 system HD 104237.
These measurements were carried out using high spectral resolution observations
obtained with the High Accuracy Radial velocity Planet Searcher in polarimetric
mode, installed at the ESO La Silla 3.6 m telescope. In agreement with previous
studies of Herbig Ae stars, the longitudinal magnetic field in the primary is
rather weak, ranging from 47 G to 72 G. The secondary component possesses a
variable, much stronger magnetic field, up to 600 G, as expected for a
magnetically active T Tauri star. We estimated the rotation period of the
primary, $P_{\mathrm{rot}}=4.33717\pm0.00316$ d ($=104\pm0.08$ h), from metal
line equivalent width variations.
