\begin{table}%t5
\caption{Comparison of elemental abundances from different sources.}                                 \label{comp_abund}
%\centerline
{
\begin{tabular}{lcclllll}
\hline
\hline
  Object             & He     & N      &  O	& Ne      & S     & Cl 	 & Ref.    \\
\hline
\object{NGC~1535}    &  11.02 & 7.11   & 8.41   &  7.86   &       &	 &  [1]\\
                     &  10.96 &        & 8.58   &  7.97   &       &	 &  [2] \\
                     &  10.99 & 7.63   & 8.51   &  7.89   &       &	 &  [3] \\
\\
\object{NGC~2438}    &  11.05 & 8.10   & 8.63   & 8.11    & 6.69  & 5.04 &  [1]\\
                     & 11.11  & 8.24   & 8.68   &         &       &      &  [5] \\
                     & 11.15  & 8.00   & 8.38   & 7.60    & 6.51  &      &  [6]\\
\\
\object{NGC~2440}    & 11.10  & 8.79   & 8.49   & 8.01    & 6.44  & 4.76 &  [1]\\
                     & 11.18  & 9.09   & 8.82   & 8.14    &       &	 &  [2] \\
                     & 11.11  & 8.71   & 8.64   & 8.13    & 6.67  &      &  [6]\\ 
		     & 11.08  & 8.98   & 8.64   & 7.95    & 6.30  & 5.11 &  [7]\\ 
		     & 11.09  & 8.26   & 8.39   & 8.04    &       &      &  [8] \\
\\
\object{NGC~3132}    & 11.10  & 8.46   & 8.77   &  8.49   &  7.06 & 5.42 &  [1]\\
                     & 11.11  & 8.68   & 8.98   &  8.56   &       &      &  [2]\\
		     & 11.08  & 8.37   & 8.82   &  8.49   & 7.02  & 5.36 &  [8]\\ 
\\
\object{NGC~3242}    & 11.02  & 7.62   & 8.49   &  7.81   &  6.41 & 4.93 &  [1]\\
                     & 11.03  & 7.71   & 8.72   &  8.03   &       &      &  [2] \\
		     & 11.00  & 7.53   & 8.52   &  7.89   &  6.38 & 4.94 &  [8] \\	     
                     & 10.96  & 7.96   & 8.64   &  8.04   &  6.51 &      &  [9] \\
		     & 10.95  & 7.91   & 8.66   &  7.85   &  6.69 & 5.06 &  [4]  \\
\\
\object{NGC~6302}    & 11.18  &  8.72  & 8.18   &  7.81  &  6.50 & 4.75 &  [1]\\
                     & 11.13  &  8.52  & 8.40   &  7.88   &  6.75 & 4.99 &  [8] \\ 
                     & 11.23  &  8.76  & 8.41   &         &  6.69 &      & [10] \\
                     & 11.26  &  8.92  & 8.70   &  7.99   &  6.80 & 5.59 & [11] \\
\\
\object{NGC~7009}    & 11.07  &  8.24  & 8.66   &  8.13  &  6.97 & 5.29 &  [1]\\
                     & 11.06  &  8.44  & 8.93   &  8.37   &       &      &  [2] \\
		     & 11.07  &  8.40  & 8.66   &  8.04   &  6.98 &      &  [6]\\ 
                     & 11.04  &  8.23  & 8.67   &  8.04   &  6.92 &      &  [12] \\ 
                     & 11.07  &  8.10  & 8.68   &  8.16   &  7.12 &      &  [13]\\
\\
Orion                & 10.99  &  7.78  & 8.72   &  7.89   & 7.17  & 5.33 &  [14] \\
Sun                  & 10.99  &  7.97  & 8.87   &  8.08   & 7.33  & 5.50 &  [15] \\
\hline
\end{tabular}}
\medskip
References:
[1]~this paper; [2]~\citet{peimbert77}; [3]~\citet{barker89}; [4]~\citet{aller83}; [5]~\citet{guerrero99};
[6]~\citet{kingsburgh94}; [7]~\citet{hyung98}; [8]~\citet{tsamis03}; [9]~\citet{barker85}; [10]~\citet{defreitas91};
[11]~\citet{aller81}; [12]~\citet{goncalves03}; [13]~\citet{barker83}; [14]~\citet{esteban98}; [15]~\citet{grevesse96}.
\end{table}