\begin{table}%t6
\caption{Published Blazhko periods from {\it some} previous data sets of RR Lyr. If error
  bars are given in the publication, they are added.  We stress that
this table is far from exhaustive, but just indicative of the likely change of the
star's Blazhko period. }  
\label{table:6}      % is used to refer this table in the text
                        % used for centering table
%\centerline
{\begin{tabular}{l c c c l}        % centered columns (4 columns)
\hline
\hline               % inserts double horizontal lines
\multicolumn{1}{l}{Reference} &
\multicolumn{1}{c}{Years} &
\multicolumn{1}{c}{$\Delta T$ (d)} &
\multicolumn{1}{l}{$P_{\rm B}$ (listed)} &
\multicolumn{1}{l}{Comments} \\
\hline                        % inserts single horizontal line
%Prager (1916) &  & &  41 & \\
Shapley (1916) & 1899-1914$^{(1)}$ &  5540 &  40 &  $P_{\rm B}\simeq 70P_0^{(2)}$\\
Bal\'asz \& Detre (1943) & 1935-1941 & 2188 & 41 & $P_{\rm B}\simeq 72.4P_0^{(3)}$\\
%2913 data points, they say 6512
Struve \& Blaauw (1948) & 1947 &  42 & 42 &$P_{\rm B}\simeq 75P_0^{(4)}$ \\
%390 data points 
Walraven (1949) & 1947 & 201 & 41$^{(5)}$ & \\
%3038 data points
Fringant (1961)$^{(1)}$ & 1949-1956 & 2500+ & 40.8$^{(1),(6)}$ & $P_{\rm B} \simeq 72 P_0$\\
%250 data points
Preston et~al. (1965) &1961-1964 & 1026 (Ph.), 855 (Sp.)$^{(7)}$& 40.8$^{(8)}$   & \\ 
Detre \& Szeidl (1973)  & 1899-1972 & 25000+ & 41$^{(9)}$ & \\
Belserene (1999) & 1993-1994 & 477 & 39.2 & \\
%161 data points
Smith et~al. (2003) & 1996 & 77 & 40 ($\pm$ 1) & \\
%2461 data points
%Chadid et~al. (1999)$^{(3)}$ & 1994, 1996, 1997 & & & & \\
This paper & 2003-2004 & 421 & 38.8 $\pm$ 0.1 &  \\
%14702 data points
\hline
\end{tabular}}
$^{(1)}$  Result also based on older data sets by other observers.   \\
%\multicolumn{5}{l}{\footnotesize{
$^{(2)}$  Derived from the variations in time of the median magnitude. Also
    mention of a longer cycle of 16.5 years.   \\
%\multicolumn{5}{l}{\footnotesize{
$^{(3)}$  The authors write that the light curve variations of RR Lyr can most
easily be discribed by a superposition of two pulsations with the 
% \multicolumn{5}{l}{\footnotesize{
period
$P_0=0.567$ d and $P_1=0.559$ d.  \\
%\multicolumn{5}{l}{\footnotesize{
$^{(4)}$  From the variation in the radial velocity curve.   \\
%multicolumn{5}{l}{\footnotesize{
$^{(5)}$  An additional period is mentioned, about three times longer
    than $P_{\rm B}$.   \\
%\multicolumn{5}{l}{\footnotesize{
$^{(6)}$  The author mentions four
    periods of RR Lyr: $P_0=0.5668$ d, $P_{\rm B}=40.8$ d, $P_3 \simeq 3P_{\rm B}$, and $P_4$
of about 10 years.   \\
%\multicolumn{5}{l}{\footnotesize{
$^{(7)}$  Simultaneous spectroscopic and photometric data.   \\
%\multicolumn{5}{l}{\footnotesize{
$^{(8)}$  Reference to Fringant's (1961) elements.   \\
%\multicolumn{5}{l}{\footnotesize{
$^{(9)}$  First mention of RR Lyr's "4-year cycle''.  The authors report that
    during one and the same
"4-year cycle'' $P_{\rm B}$ remained constant. 
\end{table}