  We report on the detection of a rapidly variable narrow Fe K$\alpha$ line in
Mkn 841. The source has been observed two times by XMM-Newton and
simultaneously with BeppoSAX. The two observations, of about 10ks long each,
were separated by $\sim$ 15 hours. The line flux reaches a maximum during the
first observation and is significantly reduced in the second one. The continuum
shape and flux, instead, keep roughly constant between the two pointings. Such
rapid variability of a narrow (unresolved by the XMM-pn instrument) line has
never been reported in the past. These results are not easily explained in the
standard cold reflection model where the narrow line component is supposed to
be produced far from the primary X-ray source (e.g. from the torus) and is thus
not expected to vary rapidly. Different interpretations are discussed.
