The 2007 outburst of the X-ray binary XTE?J1856+053

Aims. 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.Methods. 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.Results. 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 kTin= 0.75(?0.01)?keV and foreground absorption NH= 4.5(?0.1)?1022?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?. 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?h.