  We present first specific-heat data of the alloy Yb(Rh_(1-x)Co_x)_2Si_2 at
intermediate Co-contents x=0.18, 0.27, and 0.68. The results already point to a
complex magnetic phase diagram as a function of composition. Co-doping of
YbRh_2Si_2 (T_N^{x=0}=72 mK) stabilizes the magnetic phase due to the volume
decrease of the crystallographic unit cell. The magnetic phase transitions are
clearly visible as pronounced anomalies in C^{4f}(T)/T and can be suppressed by
applying a magnetic field. Going from x=0.18 to x=0.27 we observe a change from
two mean-field (MF) like magnetic transitions at T_N^{0.18}=1.1 K and
T_L^{0.18}=0.65 K to one sharp \lambda-type transition at T_N^{0.27}=1.3 K.
Preliminary measurements under magnetic field do not confirm the field-induced
first-order transition suggested in the literature. For x=0.68 we find two
transitions at T_N^{0.68}=1.14 K and T_L^{0.68}=1.06 K.
