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The high-frequency-peaked BL Lac, MS0205.7+3509 was observed twice withXMM-Newton. Both X-ray spectra are synchrotron-dominated, with mean 0.2--10keVfluxes of 2.80+/-0.01 and 3.34+/-0.02 E-12 erg/cm^2/s. The X-ray spectra arewell fit by a power-law with absorption above the Galactic value, however noabsorption edges are detected, implying a low metallicity absorber (Z_\sun =0.04(+0.03)(-0.01)) or an absorber with redshift above one (best-fit z=2.1 foran absorber with solar abundances). In either case the absorbing column densitymust be ~9E21 cm^-2. A new optical spectrum is presented, with a MgIIabsorption doublet detected at z=0.351, but no other significant features. Theoptical spectrum shows little reddening, implying a low dust to gas ratio inthe absorber. MS0205.7+3509 must therefore be viewed through a high columndensity, low-metallicity gas cloud, probably at z=0.351 and associated with thegalaxy that has been shown to be within ~2" of the BL Lac.Comment: 5 pages, 4 figures, accepted for publication in A&

XMM-Newton observations of the BL Lac MS 0205.7+3509: A dense, low-metallicity absorber