\begin{table}%t6
\caption{Best-fit parameter values and $1\sigma$ errors for the BBN
  and CMB constraints on $\frac{\Delta\alpha}{\alpha_0}$.}
\label{resultados-sum}
%\centering
\small
\begin{tabular}{cc}
\hline
\hline
%& \\[-8pt]
Group of data & $\frac{\Delta \alpha}{\alpha_0} \pm \sigma$ \\ 
%& \\[-8pt]
\hline 
BBN& $-0.020 \pm 0.007$\\  
CMB& $-0.015 \pm 0.012$ \\ 
\hline
\end{tabular}
\end{table}