  We report the frequency measurement with an accuracy in the 100 kHz range of
several optical transitions of atomic Sr : $^1S_0- ^3P_1$ at 689 nm, $^3P_1-
^3S_1$ at 688 nm and $^3P_0- ^3S_1$ at 679 nm. Measurements are performed with
a frequency chain based on a femtosecond laser referenced to primary frequency
standards. They allowed the indirect determination with a 70 kHz uncertainty of
the frequency of the doubly forbidden $5s^2^1S_0- 5s5p^3P_0$ transition of
$^{87}$Sr at 698 nm and in a second step its direct observation. Frequency
measurements are performed for $^{88}$Sr and $^{87}$Sr, allowing the
determination of $^3P_0$, $^3P_1$ and $^3S_1$ isotope shifts, as well as the
$^3S_1$ hyperfine constants.
