\begin{table}%t3
%\centering
\par
\caption { Number of stars observed ($N$) and mean internal errors 
           ($\sigma$) as a function of apparent visual magnitude.}
\begin {tabular} {ccccccccccc}
\hline
%\cline{2-5}
\hline
 $V$ range & $V$ && $(b-y)$ && $m_1$ && $c_1$ && $H_{\beta}$ \\
\hline
 & $N$&$\sigma$ & $N$&$\sigma$ & $N$&$\sigma$ & $N$&$\sigma$ &$N$&$\sigma$ \\
\hline
 8-- 9 &    3 & 0.011 &    3 & 0.068 &    3 & 0.012 &    3 & 0.042 &    2 & 0.007 \\
 9--10 &    8 & 0.010 &    8 & 0.030 &    8 & 0.041 &    8 & 0.043 &    4 & 0.014 \\
10--11 &   18 & 0.008 &   18 & 0.007 &   16 & 0.014 &   14 & 0.014 &    8 & 0.039 \\
11--12 &   36 & 0.010 &   36 & 0.013 &   33 & 0.016 &   32 & 0.022 &   19 & 0.026 \\
12--13 &  102 & 0.011 &  102 & 0.014 &  100 & 0.025 &   95 & 0.026 &   65 & 0.018 \\
13--14 &  167 & 0.009 &  167 & 0.011 &  165 & 0.023 &  158 & 0.025 &   86 & 0.018 \\
14--15 &  300 & 0.009 &  299 & 0.014 &  297 & 0.019 &  292 & 0.022 &  151 & 0.028 \\
15--16 &  471 & 0.009 &  471 & 0.016 &  467 & 0.024 &  458 & 0.031 &  216 & 0.024 \\
16--17 &  833 & 0.014 &  833 & 0.026 &  799 & 0.038 &  761 & 0.047 &  355 & 0.027 \\
17--18 & 1110 & 0.021 & 1110 & 0.036 &  945 & 0.050 &  843 & 0.058 &  317 & 0.037 \\
18--19 & 1140 & 0.019 & 1139 & 0.038 &  882 & 0.045 &  734 & 0.055 &   36 & 0.036 \\
19--20 & 1204 & 0.012 & 1204 & 0.029 &  991 & 0.036 &  565 & 0.056 &      &       \\
20--21 & 1119 & 0.017 & 1119 & 0.037 &  867 & 0.055 &  143 & 0.072 &      &       \\
21--22 & 1007 & 0.045 & 1007 & 0.064 &  387 & 0.090 &    5 & 0.092 &      &       \\
22--23 &  252 & 0.090 &  252 & 0.121 &   32 & 0.139 &      &       &      &       \\
\hline
Total & 7770 &       & 7768 &       & 5992 &       & 4111 &       & 1259 &       \\
\hline
\end {tabular}
%%\end{center}
\par
\label{error}
\end {table}
\par
\addtocounter{figure}{1}
\par
\begin{figure}%f2
\htmlimage{scale=2.5, external, thumbnail=0.4}
%\centering
\par
\includegraphics[width=8.8cm,clip]{1333fig2.eps}
\caption{Internal errors of magnitude and colours as a function of the          apparent visual magnitude, $V$, 
for all stars in the cluster region. The
structure in the magnitude dependence is owed to the mosaic of images
from different nights and different telescopes having different limiting
magnitude.}
\label{errby}
\end{figure}
\par
Table~4 lists the $u,v,b,y,$ $H_{\beta}$ data for all 7842 stars in a
region of $65^{\prime} \times 40^{\prime}$ around the
open cluster NGC~1817 (Fig.~\ref{map}). Star centres are given as frame ($x, y$)
and equatorial ($\alpha_{\rm J2000}$, $\delta_{\rm J2000}$) coordinates. 
Given the large 
area coverage and depth of our photometry, a new numbering system 
for stars in this field is introduced. 
An identification number was assigned to each star following 
the order of increasing right ascension.
Column~1 is the ordinal star number; Cols.~2 and 3
are $\alpha_{\rm J2000}$ and $\delta_ {\rm J2000}$;
Cols.~4 and 5 are the respective $x$, $y$ coordinates in arcmin;
Cols.~6 and 7 are the $(b-y)$ and its error,
8 and 9 the $V$ magnitude and its error, 10 and 11 the $m_1$ and its error, 
12 and 13 the $c_1$ and its error, 
and 14 and 15 the $H_{\beta}$ and its error. 
%Column 16 distinguishes/labels the stars selected as candidate members
%(Sect. 3.1) from those considered non-members. 
In Col.~16, stars considered candidate members (Sect.~3.1) are labelled
 "M'', while those classified as non-members show the label "NM''.
\par
\addtocounter{table}{1}
\par
    The cross-identification of stars in common
    with the astrometry (Paper~II), WEBDA
({\footnotesize{\tt htpp://obswww.unige.ch/ WEBDA}}), 
Hipparcos (ESA, \cite{esa}), Tycho-2 (H\a{o}g et~al.\ \cite{tyc2}) 
and USNO-2 (Monet et~al.\ \cite{USNO2}) catalogues
    is provided in Table~5. 
\addtocounter{table}{1}
\par
\subsection{Comparison with previous photometry}
\par
	Only four stars in the field of NGC~1817 have been previously studied 
using Str\"omgren photometry, in the range of $V=9.5$ to 12.9. 
Maitzen et~al.\ (\cite{Maitz}) performed a 
photometric search for Ap stars among Blue Stragglers in open clusters,  
with mean errors of less than 0.01.
Mean differences in the sense ours minus others are:
0.03($\sigma=0.04$) in $V$, 0.02 ($\sigma=0.04$) in $b-y$,
$-$0.03 ($\sigma=0.07$) in $m_1$ and 0.00 ($\sigma=0.03$) in $c_1$
for the 4 stars in common.
\par
	On the other hand, the $V$ magnitude derived from the $y$ filter
can be compared with the published broadband data. 
	The recent study of radial velocities of red giants in the area 
by Mer03 included photoelectric photometry of 25 stars. 
We have 22 common stars with the photoelectric photometry of HH77 
and 19 common stars with Purgathofer (\cite{Purga}), up to $V=16$.
	The corresponding mean differences in $V$, in the sense ours minus others
are: $-$0.01(0.03),$-$0.01(0.04) and $-$0.01(0.05), respectively. 
Transformation between
$B-V$ and $b-y$ from several authors (see Moro \& Munari \cite{ADPS}) fails 
to cover the whole range under study. Making use of merged broadband photometry,
we can find a linear relation between the two indices: 
$B-V=(1.719\pm0.035)(b-y)-(0.170\pm0.024)$, $N=62$. 
The standard deviation of 
the residuals about the mean relation is 0.055, where the typical 
uncertainty in $B-V$ is 0.02 and in $b-y$ is 0.014.
\par
%_____________________________________________________________________
\par
\section{Colour--magnitude diagrams}
\par
        We use the $V$ vs.\ $(v-y)$ colour--magnitude diagram 
for our study. As Meiborn (\cite{Mei00}) stated, the colour-magnitude 
diagram based on this
colour index defines the main-sequence of a cluster significantly better 
than the traditional $V$ vs.\ $(b-y)$ diagram (Fig.~\ref{HR} left and centre).
The fact that the reddening vector runs parallel 
to the cluster sequence in this diagram
allows a much better separation of cluster and field stars. 
\par
        The observational colour-magnitude diagram for all the stars
in the studied area 
(Fig.~\ref{HR}) displays a fairly well defined main sequence. Specially 
outstanding against the field background is the red giant clump and the 
main sequence between 0.8$<(v-y)<$1.8. 
\par
\begin{figure}%f3
\htmlimage{scale=2.1, external, thumbnail=0.4}
%\centering

\includegraphics[width=16cm,clip]{1333fig3.eps}
\caption{The colour--magnitud diagram of the NGC~1817 area.
Empty circles in the right figure are the astrometric members from Paper~II. 
Black line is a shifted ZAMS, with the chosen margin for candidate members
($V+0.5$,$V-1$) in white lines. See text for details.}
\label{HR}
\end{figure}
\par
\subsection{Selection of candidate member stars}
\par
	The first step in the study of an open cluster is the determination
of its member stars. 
A selection of probable member stars 
up to the limiting magnitude of our sample
can be obtained
combining astrometric with photometric criteria. 
Unfortunately, proper motions 
(Paper~II) and radial velocities
(Mer03) 
are only available for the 
brightest stars in the area. 
Photometric 
measurements help to reduce the possible field contamination in the
proper motion membership -- among bright stars --,
as well as to enlarge the selection of members towards faint magnitudes.  
\par
\begin{figure}%f4
\htmlimage{scale=2.5, external, thumbnail=0.4}
\resizebox{8.8cm}{!}{\includegraphics{1333fig4.eps}}
\caption{The colour--colour diagrams of NGC~1817.
           Empty circles denote candidate members of NGC~1817,
chosen with astrometric and non-astrometric criteria as 
explained in Sect.~3.1.
           The thick line is the standard relation shifted 
 $E(b-y)$ = 0.19 when necessary.} 
\label{colour}
\end{figure}
\par
Among those astrometric member stars, we find 13 stars that are not 
compatible with 
the sequence of the cluster outlined in the colour-magnitude diagram.
Radial velocity information is also taken into account,
to reject stars with radial velocities incompatible with membership
and to include stars considered members by Mer03 and compatible
with membership according to our photometric data.
Our astrometric study -- based on proper motion from plates --, and thus its
segregation of member stars, has a limiting magnitude of $V=14.5$. 
From this point down to our photometric limit, $V=22$ we construct a ridge line
following a fitting of the   
observational ZAMS (Crawford \cite{Craw75}, \cite{Craw78}, \cite{Craw79};
Hilditch et~al.\ \cite{Hil83}; Olsen \cite{Ols84})  
in the $V - (v-y)$ diagram. 
A selection of stars based on the distance to this ridge line is then
obtained. The chosen margin for candidates includes all the stars 
between $V+0.5$ and $V-1$ from the ridge line, as shown in the
right panel of Fig.~\ref{HR}. The margins were chosen to account for
observational errors and the presence of multiple stars.
\par
This preliminary photometric selection is refined in the colour-colour 
diagrams (Fig.~\ref{colour}) with the help of the standard relations from the
same authors.  
A final selection of 1592 stars in the area is plotted in Fig.~\ref{colour}
as empty circles in the $[m_1] - [c_1]$, $m_1 - (b-y)$, $c_1 - (b-y)$ and 
$[c_1] - {H}_{\beta}$ diagrams. 
\par
\section{Physical parameters of the cluster}
\par
Narrow and intermediate passband photometry  
constitutes a useful technique for classification of the stars (Stromgren
\cite{Strom}; Philip et~al.\ \cite{Phil}).
The stars of the area selected as possible cluster members were
classified into photometric regions and their physical parameters were determined. 
The algorithm uses 
$uvby$--${H}_{\beta}$ photometry and standard relations among colour indices
for each of the photometric regions of the HR diagram. 
Masana et~al.\ (\cite{Mas}) provide improvements to the algorithm 
classification and parametrization described in Jordi 
et~al.\ (\cite{Jordi97}) and Figueras et~al.\ (\cite{Fig}). Improvements
concern the inclusion of a grid of temperatures and gravities dependent on 
metallicity, the determination of masses and radii as well as the detection
of peculiar stars.
\par
        Absolute magnitude, effective temperature and gravity as well 
as the corresponding reddening, distance modulus, metallicity and a raw
spectral type and luminosity class are calculated 
for each star. Typical errors are 0.25~mag in $M_V$, 0.15~dex in [Fe/H],
270~K in $T_{\rm eff}$, 0.18~dex in $\log g$ and 0.015~mag in $E(b-y)$.
Even if the result for the physical parameters for a given star 
would be inaccurate -- mainly due to 
peculiarity, emission lines or binary character --,
the high proportion of "normal'' stars 
makes it possible to ascertain the physical parameters of the cluster. 
\par
	Only 525 stars among the 1592 candidate members have $H_{\beta}$
measurements. So the computation of physical parameters is only possible
for that subset. Red giant stars are outside of the validity of the 
calibrations. 
Excluding peculiar stars and those with inconsistence among their
photometric indices, we apply
an average with a 2$\sigma$ clipping to that subset and   
only 264 stars remain (Fig.~\ref{Vvyby1}).
We found a reddening value of $E(b-y)~=0.19 \pm 0.05$
(corresponding to $E(B-V)~=0.27$)
and a distance modulus of $V_0-M_V~=10.9 \pm 0.6$. 
Metallicity is better calculated studying only the 196 F and G stars in 
our sample, inside the validity range of the calibrations of 
Schuster \& Nissen (\cite{Schu},
see Masana et~al.\ \cite{Mas} for more details). 
We found a value of [Fe/H]~=~$-0.34 \pm 0.26$. 
        Figure~\ref{pf} shows these results. 
\par
	Our results are consistent with previous results. 
HH77 give a reddening value of $E(B-V)~=0.28 \pm 0.03$ and a 
distance modulus of $11.3\pm 0.3$.
Friel \& Janes (\cite{Friel93}) give a value of [Fe/H]~$= -0.39 \pm 0.04$ 
($N=4$) on the basis
of moderate-resolution spectra, while Taylor (\cite{Tay}) reexamines the errors 
in these data and concludes a lower value of [Fe/H]~$= -0.42 \pm 0.07$.  
Friel et~al.\ (\cite{Friel02}) give a recalibrated value of 
[Fe/H]~$= -0.29 \pm 0.05$ based on 3 stars. 	
Twarog et~al.\ (\cite{Twa}) find an $E(B-V)~=0.26$, an [Fe/H]$~= -0.27 \pm 0.02$ 
and $V_0-M_V~=11.34$ based on main-sequence fitting.  
Dutra \& Bica (\cite{Dutr}) using DIRBE/IRAS 100 $\mu {\rm m}$ dust emission 
integrated throughout the Galaxy derive a value of $E(B-V)_{\rm FIR}$~=~0.33.
\begin{figure}%f5
\htmlimage{scale=2.5, external, thumbnail=0.4}
\resizebox{8.8cm}{!}{\includegraphics{1333fig5.eps}}
\caption{Colour--magnitude diagram with the subset of 264 stars used to
derive the physical parameters in empty circles, the
red giants in full circles, 
blue stragglers in diamonds and $\delta$ Scuti in crosses. Field stars are in dots.
The thick line is the ZAMS, shifted in extinction and distance modulus. 
}
\label{Vvyby1}
\end{figure}
\par
\begin{figure}%f6
\htmlimage{scale=2.5, external, thumbnail=0.4}
\resizebox{8.8cm}{!}{\includegraphics*{1333fig6.eps}}
\caption{The histograms of the distance modulus, reddening and 
metallicity of the selected member stars of NGC~1817 (white), 
and the corresponding ones for the stars in the NGC~1807 area (dashed).}
\label{pf}
\end{figure}
\par
	The recent publication by Clem et~al.\ (\cite{Clem}) 
of empirically constrained colour-temperature relations in the Str\"omgren
system allows the transformation of isochrones from the theoretical to
the observational colour--magnitude diagram. Several sets of isochrones
have been used to analyze our results.
%makes possible an isochrone fitting to our results. 
The best fitting is
found for the Schaerer et~al.\ (\cite{Scha}) isochrones. Figure~\ref{iso}
shows isochrones of $Z=0.008$ shifted by a reddening of 0.19 and an apparent 
distance modulus of 11.6. We found a best estimation of the
age of $\log t = 9.05 \pm 0.05$.  
	HH77 give a Hyades-age for this cluster: 0.8 Gyr ($\log t= 8.9$). 
But a recent determination of ages of old open clusters 
(Salaris et~al.\ \cite{Sal}) gives an age of 
$1.12 \pm 0.18$ Gyr based on the morphological age index provided by 
Janes \& Phelps (\cite{JP94}) but on a new highly homogeneous and 
reliable calibration in terms of absolute ages. 
Our age determination, 1.1~Gyr, agrees very well.
\par
\begin{figure}%f7
\htmlimage{scale=2.2, external, thumbnail=0.4}
%\begin{center}
\par
\resizebox{12cm}{!}{\includegraphics{1333fig7.eps}}
\caption{Geneva isochrones (Schaerer et~al.\ \cite{Scha})
of $\log t$ 8.9, 9.0 and 9.1 and $Z=0.008$.
The adopted reddening and apparent distance modulus are 0.19 and 11.6,
respectively.
           Empty circles denote candidate members of NGC~1817.} 
\label{iso}
\end{figure}
\par
There are seven known blue stragglers in our photometry from the 
catalogue of blue stragglers in open clusters 
(Ahumada \& Lapasset \cite{Ahu95}). But two of them (HH2090, HH1073) are not 
proper motion members. 
From a sample of 18 clusters, Wheeler (\cite{Wheel})
found a ratio of the number of stragglers to giants of
from 1/3 to 1/4. Having 39 confirmed red giants in the cluster 
(Mer03) we should expect between 10 and 13
blue stragglers.
The location in our colour-magnitude diagram of six blue stars 
that are astrometric members according to Paper~II lead us to 
classify them also as blue stragglers.
Their photometry and astrometric segregation    
from Paper~II are listed in Table~\ref{BlueS}.
\par
\begin{table}%t6
%\centering
\par
\caption {Blue Stragglers in NGC~1817, with their membership 
from Paper~II. The first seven are from the catalog of 
Ahumada \& Lapasset (\cite{Ahu95}). The last six have been classified
in this work.	 
         }
\label{BlueS}
\begin {tabular} {cccccccccc}
\hline \hline
  Id.  & HH & $(b-y)$ & $V$ &  $P$  \\
\hline
\par
7091 & 2045 &  $0.321 \pm 0.0059$ & $12.305 \pm 0.0176$ & M  \\
 157 & 2090 &  $0.209 \pm 0.0097$ & $13.112 \pm 0.0000$ & NM \\
 389 & 2072 &  $0.188 \pm 0.0011$ & $12.195 \pm 0.0009$ & M  \\
 387 & 2110 &  $0.184 \pm 0.0051$ & $11.762 \pm 0.0056$ & M  \\
7467 & 2020 &  $0.221 \pm 0.0126$ & $12.896 \pm 0.0074$ & M \\
7746 & 1073 &  $0.421 \pm 0.0147$ & $12.270 \pm 0.0123$ & NM \\
7310 & 3001 &  $0.197 \pm 0.0210$ & $13.816 \pm 0.0309$ & M  \\
1126 &      &  $0.174 \pm 0.0027$ & $12.486 \pm 0.0029$ & M  \\
 390 &      &  $0.379 \pm 0.0036$ & $12.157 \pm 0.0048$ & M  \\
 388 &      &  $0.379 \pm 0.0101$ & $11.914 \pm 0.0035$ & M  \\
  45 &      &  $0.427 \pm 0.0014$ & $11.498 \pm 0.0172$ & M  \\
6161 &      &  $0.259 \pm 0.0023$ & $10.514 \pm 0.0027$ & M  \\
7332 &      &  $0.183 \pm 0.1662$ & $14.169 \pm 0.0965$ & M  \\
\par
\hline
\end{tabular}
\end{table}