  Quasars selected in optical surveys by means of their blue or UV excess are
known to be strong emitters in the X-rays, except for Broad Absorption Line
(BAL) objects. In this paper we study the X-ray emission of quasars selected
through their emission line spectrum rather than their blue excess. X-ray data
are obtained by cross-correlating two optical samples (the Hamburg Survey
Catalogue and the Palomar Transit Catalogue) with the WGA catalogue of ROSAT
observations. We find that a significant fraction of objects are strongly
underluminous in the X-rays. We discuss the physical nature of these sources
and we propose an interpretation for their optical and X-ray properties based
on a dust-poor and gas-rich absorber. Our results suggest the existence of a
population of AGNs having a type 1 optical spectrum and a type 2 X-ray emission
