  We report the results of quasi-simultaneous observations of the bright quasar
3C 273 using the BeppoSAX Low-Energy Concentrator Spectrometer (LECS) and the
ASCA Solid-State Imaging Spectrometer (SIS). These observations are part of an
intercalibration programme and demonstrate good agreement between both
instruments in the common energy range 0.5-10 keV. In particular, the
absorption feature discovered by BeppoSAX/LECS at $\sim$ 0.5 keV is
consistently seen by ASCA/SIS. We present the most accurate measurement yet of
the spectral shape of this absorption feature and provide constraints on its
physical interpretation. A self-consistent warm absorber model in
photoionization equilibrium only gives an adequate spectral fit if a very high
velocity inflow is allowed for the warm absorber (v $\sim$ 0.36 c). The
ionization parameter and the column density of the warm absorber are then
$\sim$ 1 and $\sim$ 1.3e21 cm^-2, respectively. If the differences in best-fit
neutral absorption column density between the LECS and each SIS are taken as
estimates of the systematic spectral uncertainties, then these are < 0.8e20 and
< 2.e20 cm^-2 (at 90 % confidence) for SIS0 and SIS1, respectively.
