We present combined optical/near-IR photometry ( BVIK ) for six open clusters - M35 , M37 , NGC 1817 , NGC 2477 , NGC 2420 , and M67 .
The open clusters span an age range from 150 Myr to 4 Gyr and have metal abundances from [ Fe/H ] = –0.3 to +0.09 dex .
We have utilized these data to test the robustness of theoretical main sequences constructed by several groups as denoted by the following designations - Padova , Baraffe , Y ^ { 2 } , Geneva , and Siess .
The comparisons of the models with the observations have been performed in the [ M _ { V } , ( B - V ) _ { 0 } ] , [ M _ { V } , ( V - I ) _ { 0 } ] , and [ M _ { V } , ( V - K ) _ { 0 } ] colour-magnitude diagrams as well as the distance-independent [ ( V - K ) _ { 0 } , ( B - V ) _ { 0 } ] and [ ( V - K ) _ { 0 } , ( V - I ) _ { 0 } ] two-colour diagrams .
We conclude that none of the theoretical models reproduce the observational data in a consistent manner over the magnitude and colour range of the unevolved main sequence .
In particular , there are significant zeropoint and shape differences between the models and the observations .
We speculate that the crux of the problem lies in the precise mismatch between theoretical and observational colour-temperature relations .
These results underscore the importance of pursuing the study of stellar structure and stellar modelling with even greater intensity .