  We present specific-heat measurements in the vicinity of the
antiferromagnetic phase transition on single crystals of the alloy
Yb(Rh_{1-x}Co_x)2Si2 for x<= 0.38. This study was motivated by the violation of
critical universality in the undoped YbRh2Si2 (Krellner et al., Phys. Rev.
Lett. 102, 196402) where we have found a large critical exponent a=0.38. For
Co-doped samples we observe a drastic change of the critical fluctuations
resulting in a negative a, explainable within the universality classes of phase
transitions. The development of a under chemical pressure gives strong
indication that the violation of critical universality in YbRh2Si2 is due to
the nearby quantum critical point.
