\begin{table}%t4
  \caption{Observed orbital period modulations in CVs.\label{tab:modcvs}}
%\centerline
 {  \begin{tabular}{lcccl}
    \hline\hline
 Object & $P_{{\rm orb}}$ & $P_{{\rm mod}}$ & $\Delta P/P$       & Ref.\\
        & (h)            & (yr)            & ($\times 10^{-6}$) &     \\ 
    \hline
    V4140~Sgr & 1.47 &  6.9 & 0.93 & 1 \\ 
    V2051~Oph & 1.50 &   22 & 0.30 & 1 \\
    OY~Car    & 1.51 &   35 & 0.52 & 2 \\ 
    EX~Hya    & 1.64 & 17.5 & 0.55 & 3 \\
    HT~Cas    & 1.77 &   36 & 0.44 & This work \\
    Z~Cha     & 1.79 &   28 & 0.85 & 4 \\
    IP~Peg    & 3.80 &  4.7 & 8.00 & 5 \\
    U~Gem     & 4.17 &    8 & 3.00 & 6 \\
    DQ~Her    & 4.65 & 13.7 & 2.00 & 7 \\
    UX~UMa    & 4.72 & 7.1,~10.7,~30.4$^{\ast}$ & 2.60 & 8 \\
    T~Aur     & 4.91 &   23 & 3.80 & 9, this work \\
    EX~Dra    & 5.03 &    5 & 7.90 & 10 \\
    RW~Tri    & 5.57 & 7.6,~13.6$^{\ast}$ &2.10 & 11 \\  
    AC~Cnc    & 7.21 & 16.2 & 5.50 & 12 \\
    \hline
    \end{tabular}}
\par
    \smallskip
      $^{\ast}$ Multiperiodic (the best determined
    periodicities are considered). 
       {References}: (1) Baptista et~al. (\cite{Baptista03}); (2) 
      Greenhill et~al. (2006); 
      (3)~Hellier \& Sproats (\cite{Hellier92}); (4) Baptista et~al. 
      (\cite{Baptista02}); (5) Wolf et~al. (\cite{Wolf93}); (6) Warner (\cite{Warner88}); (7) 
    Zhang et~al.       (\cite{Zhang95}); (8) Rubenstein 
       et~al. (\cite{Rubenstein91}); (9) Beuermann \& Pakull (\cite{Beuermann84});
      (10) Shafter \& 
      Holland (2003); (11) Robinson et~al. (\cite{Robinson91});
    (12) Qian et~al. (\cite{Qian07}b). 
  \end{table}