Detection of a spiral lens galaxy and optical variability in the gravitational lens system B1600+434 ?

The gravitationally lensed quasar B1600+434 (z = 1:61, mV =2 1 :6) has been observed at the 2.56m Nordic Opti- calTelescope(NOT).InthisLetterwereportthediscoveryofan edge-on late-type galaxy located between the two lensed com- ponents (separation 1: 4), close to the fainter image. The galaxy photometry indicates that its redshift is approximately 0:4. We detect a large colour difference between the two images due to signicantobscurationofthefaintimage.Theestimatedamount ofabsorptionasafunctionofcolourindicatesthattheextinction may be due to dust in the lensing galaxy. We also present evi- dence of flux variability in B1600+434 with a detected change of 0.25 mag in one year. The theoretically expected time de- lay is of the order of one month and so the system may be an interesting object for determining the Hubble constant.