\begin{table}%t6
\caption{\label{tab6}Metallicity gradients.}
%\centering
\small
\begin{tabular}{lrrr}
\hline\hline
X& $N_{\rm PNe}$ & $\Delta$log(X/H)/$\Delta R_{\rm G}$ & $A$(X)$_0$\\
(1)& (2)& (3)& (4)\\
\hline
\par
%He&	15&	0.13$\pm0.05$&	$\dots$& $\dots$\\
%N&	16&  3.8$\pm 1.9\times10^{-5}$&$\dots$&$\dots$\\
O&	19&   $-0.055\pm0.02$& $8.72\pm0.18$ \\
Ne&	6&	   $-0.047\pm0.03$& $8.18\pm0.27$ \\
S&	16&     $-0.069\pm0.01$& $7.27\pm0.10$\\
%Ar&	4&      1.2$\pm0.8\times10^{-6}$&$\dots$&$\dots$\\
\hline
X& $N_{\rm \hii~~reg.}$&$\Delta$log(X/H)/$\Delta R_{\rm G}$& $A$(X)$_0$\\
O&  31&   $-0.093\pm0.02$ & $9.37\pm0.24$ \\
\hline
\end{tabular}
\end{table}