Further evidence that 1RXS J170849.0-400910 is an anomalous X-ray pulsar

We report the results of two ROSAT HRI observations of the recently discovered similar to 11 s X-ray pulsar 1RXS J170849.0-400910. A refined position with a smaller error radius (10 " uncertainty) and a new spin-period measurement were obtained. These results allowed us to derive a period derivative of similar to 7 x 10(-4) s yr(-1) and to perform a photometric and spectroscopic study of the possible optical counterparts of the source. The limits derived from the optical-to-X-ray flux ratio exclude the presence of a massive OB companion. These findings, together with the nearly constant X-ray flux across observations spanning 3 years, strongly support the inclusion of this similar to 11 s pulsar in the class of anomalous X-ray pulsars.