We constrain the mass-to-light ratios , gas mass fractions , baryon mass fractions and the ratios of total to luminous mass for a sample of eight nearby relaxed galaxy groups and clusters : A262 , A426 , A478 , A1795 , A2052 , A2063 , A2199 and MKW4s .
We use ASCA spatially resolved spectroscopic X-ray observations and ROSAT PSPC images to constrain the total and gas masses of these clusters .
To measure cluster luminosities we use galaxy catalogs resulting from the digitization and automated processing of the second generation Palomar Sky Survey plates calibrated with CCD images in the Gunn-Thuan g , r , and i bands .

Under the assumption of hydrostatic equilibrium and spherical symmetry , we can measure the total masses of clusters from their intra-cluster gas temperature and density profiles .
Spatially resolved ASCA spectra show that the gas temperature decreases with increasing distance from the center .
By comparison , the assumption that the gas is isothermal results in an underestimate of the total mass at small radii , and an overestimate at large cluster radii .

We have obtained luminosity functions for all clusters in our sample .
After correcting for background and foreground galaxies , we estimate the total cluster luminosity using Schechter function fits to the galaxy catalogs .
In the three lowest redshift clusters where we can sample to fainter absolute magnitudes , we have detected a flattening of the luminosity function at intermediate magnitudes and a rise at the faint end .
These clusters were fitted with a sum of two Schechter functions .
The remaining clusters were well fitted with a single Schechter function .

Assuming H _ { 0 } = 50 ~ { } h _ { 50 } ~ { } { km } ~ { } { s } ^ { -1 } ~ { } { Mpc } ^ { -1 } , the measured mass-to-light ratios are \sim 100 ~ { } h _ { 50 } ~ { } { M } _ { \odot } / { L } _ { \odot } .
This , along with a high baryonic fraction , is indicative of a low density universe with \Omega _ { 0 } \sim 0.15 - 0.2 .