We present new long-slit William Herschel Telescope spectroscopic observations of the lens galaxy G1 associated with the double-imaged QSO 0957+561A , B .
The obtained central stellar velocity dispersion , \sigma _ { l } = 310 \pm 20 km s ^ { -1 } , is in reasonable agreement with other measurements of this dynamical parameter .
Using all updated measurements of the stellar velocity dispersion in the internal region of the galaxy ( at angular separations < 1 \farcs 5 ) and a simple isotropic model , we discuss the mass of a possible central massive dark object .
It is found that the data of Falco et al .
( 1997 ) suggest the existence of an extremely massive object of ( 0.5–2.1 ) \times 10 ^ { 10 } h ^ { -1 } M _ { \odot } ( 80 \% confidence level ) , whereas the inclusion of very recent data ( Tonry & Franx 1998 , and this paper ) substantially changes the results : the compact central mass must be \leq 6 \times 10 ^ { 9 } h ^ { -1 } M _ { \odot } at the 90 \% confidence level .
We note that , taking into account all the available dynamical data , a compact nucleus with a mass of 10 ^ { 9 } h ^ { -1 } M _ { \odot } ( best fit ) can not be ruled out .