  On 28 February 2007 a new outburst of the previously known transient source
XTE J1856+053 was detected with RXTE/ASM. We present here the results of an
XMM-Newton (0.5-10.0 keV) Target of Opportunity observation performed on 14
March 2007, aimed at constraining the mass of the compact object in this X-ray
binary and determining its main properties. The EPIC-pn camera was used in
Timing mode and its spectrum fit together with the RGS data. IR observations
with GROND at the 2.2 m telescope in La Silla provide further information on
the system. The X-ray light curve shows that both the 1996 and the 2007
outbursts had two peaks. The X-ray spectrum is well fit with a thermal
accretion disk model, with kT=0.75+/-0.01 keV and foreground absorption
N_H=4.5(+/-0.1)E22 cm**-2. The low disk temperature favours a black-hole as
accreting object, with an estimated mass in the range 1.3-4.2 M_sun. From the
IR upper limits we argue that XTE J1856+053 is a low mass X-ray binary. We
estimate the orbital period of the system to be between 3 and 12 hours.
