Limits on isocurvature fluctuations from Boomerang and MAXIMA

We present the constraints on isocurvature fluctuations for a flat universe implied by the Boomerang and MAXIMA-1 data on the anisotropy of the cosmic microwave background. Because the new data defines the shape of the angular power spectrum in the region of the first acoustic peaks much more clearly than earlier data, even a tilted pure isocurvature model is now ruled out. However, a mixed model with a sizable isocurvature contribution remains allowed. We consider primordial fluctuations with different spectral indices for the adiabatic and isocurvature perturbations, and find that the 95% C.L. upper limit to the isocurvature contribution to the low multipoles is $\ensuremath{\alpha}<~0.63.$ The upper limit to the contribution in the $l\ensuremath{\sim}200$ region is ${\ensuremath{\alpha}}_{200}<~0.13.$