= Solution
Adding the matter and ghost transformations gives
$$
\boxed{\delta_vT_{\rm tot}=v\partial T_{\rm tot}
+2(\partial v)T_{\rm tot}+\frac{D-26}{12}\partial^3v}.
$$
For the Laurent modes $L_n=\oint dz\,z^{n+1}T_{\rm tot}(z)/(2\pi i)$, the corresponding <Virasoro algebra> is
$$
\boxed{[L_m,L_n]=(m-n)L_{m+n}
+\frac{D-26}{12}m(m^2-1)\delta_{m+n,0}}.
$$
At the bosonic-string <critical dimension of string theory>, $D=26$, the total central charge and central term vanish. The total stress tensor then transforms as a genuine conformal tensor of weight two, and the quantum worldsheet gauge anomaly cancels.
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