\begin{table}%t1
\caption{\label{tab1}90~GHz observations (IRAM 30--m telescope): Precision of repeated 
measurements of secondary standards.}
\small%\centerline
{
\begin{tabular}{lccc} 
\hline
\hline\noalign{\smallskip}
Source  & $\langle T_{\rm a}^{*}\rangle $ 
& rms($T_{\rm a}^{*}$/$\langle T_{\rm a}^{*}\rangle $) & Number of \\
        & [K]    & [$\%$]   & Observ.$^{a}$ \\
\hline
NGC~7538   & 0.41(6)  & 10.0 & 351 \\
W3OH        & 0.63(4)  & 8.1  & 517 \\
NGC~7027   & 0.77(2)  & 8.2  & 488 \\
K3--50A     & 1.06(5)  & 8.7  & 451 \\
\hline\end{tabular}}
\smallskip
$^a$ For the years 1985 to 2005.
\end{table}