The designed peak luminosity of BEPC-II is $1\times 10^{33}$
cm$^{-2}s^{-1}$ at $\sqrt{s}=3.77$~GeV, i.e. the $\psi(3770)$
peak. It has reached 30\% of the design luminosity now and is
starting to deliver luminosity to BES-III for physics. Although the
physics programs at BES-III are rather rich~\cite{Asner:2008nq}, most of
the time, the machine will run at $\sqrt{s}=3.77$~GeV and 4.17~GeV
for charm physics, since the cross sections of $J/\psi$ and
$\psi(2S)$ production are large and the required statistics can be
accumulated in short time, say, one year at each energy point. The 
estimated running time of BEPC-II at $\sqrt{s}=3.77$ and
4.17~GeV is around eight years, which corresponds to an integrated
luminosity of about 20~fb$^{-1}$ at each energy point.

Data samples at $\sqrt{s}=3.77$ and 4.17~GeV can be used for
radiative return studies, for the c.m. energies of the
hadron system between the $\pi^+\pi^-$ threshold to above 2.0~GeV.
%%%TT
This will allow for measurements of the pion, kaon and proton form 
factors, as well as of cross sections for some multi-hadron final states. 
The good coverage of
the muon detector at BES-III also allows the identification of
the $\mu^+\mu^-$ final state, thus supplying a normalisation
factor for the other two-body final states.

Figure~\ref{lum3773} shows the expected luminosity at low energies
in 10~MeV bins for 10~fb$^{-1}$ data accumulated on the
$\psi(3770)$ peak. In terms of luminosity at the $\rho^0$ peak,
one can see that 10~fb$^{-1}$ of data at $\sqrt{s}=3.77$~GeV is
equivalent to 70~fb$^{-1}$ at 10.58~GeV, i.e. at the $B$ factories.

\begin{figure}
  %%\includegraphics{lum-3773.eps}
  \centerline{\psfig{file=lum-3773.eps,width=0.45\textwidth}}
\caption{Expected luminosity at low energies due to ISR for
10~fb$^{-1}$ data accumulated on the $\psi(3770)$
peak.}\label{lum3773}
\end{figure}

With Monte Carlo generated $e^+e^-\to \gamma_{ISR}\pi^+\pi^-$ data
using PHOKHARA~\cite{Rodrigo:2001kf}, after a fast simulation and
reconstruction with the BES-III software, one found the efficiency
for events at the $\rho^0$ peak to be around 5\% if one requires the
detection of the ISR photon. This is higher than the efficiency at 
BaBar \cite{wangll}. Figure~\ref{ff_bes3} shows the
signal for 10,000 generated $\pi^+\pi^-$ events. One estimates the
number of events in each 10~MeV bin to be around 20,000 at the
$\rho^0$ peak, for 10~fb$^{-1}$ of data at $\sqrt{s}=3.77$~GeV. This
is comparable with the recent BaBar results based on 232~fb$^{-1}$ of 
data at the $\Upsilon(4S)$ peak~\cite{wangll}.

\begin{figure}[htb]
  % Requires \usepackage{graphicx}
%%  \includegraphics{fpi_bes3.eps}
  \centerline{\psfig{file=fpi_bes3.eps,width=0.45\textwidth}}
\caption{Detected $\gamma_{\rm ISR}\pi^+\pi^-$ in 10000 produced
events at the $\psi(3770)$ peak. The sample is generated with
PHOKHARA.} \label{ff_bes3}
\end{figure}

The most important work related to the pion form factor
measurement is the estimate of the systematic error. Since the
cross section of good events at the $\psi(3770)$ peak is not large
(around 30~nb for the total hadronic cross section, with about 400~nb
cross section for the QED processes) 
%%%TT
compared to the highest
trigger rates at $J/\psi$ and $\psi(2S)$ peak energies, a loose
%%%TT out of question.
trigger is mandatory to allow the ISR events to be recorded. 
In principle, the trigger rate for these events could reach 100\%, 
with an allowed trigger purity of less than 20\%.

%%%TT
With enough $D\bar D$ events accumulated at the same energy, the
tracking and particle ID efficiencies can be measured with 
high precision (as has been done at CLEO-c~\cite{cleoc:2007zt}). In
addition, a huge data sample at the $\psi(2S)$ and the well measured
large branching fraction of $\psi(2S)$ transition modes, such as
$\pi^+\pi^-J/\psi$, $J/\psi\to \mu^+\mu^-$, can be used to study
the tracking efficiency, $\mu$-ID efficiency and so on. All this 
will greatly help to understand the detector performance and
to pin down the systematic errors in the form factor
measurement.

The kaon and proton form factors can be measured as well since
they are even simpler than the measurement of the pion form factor.
This will allow us to better understand the structure close
to threshold and possible existing high-mass structures.

Except for the lowest lying vector states ($\rho$, $\omega$, $\phi$), 
the parameters of other vector states are poorly known, 
and further investigations are needed. BES-III ISR analyses may
%%%TT
reach energies slightly above 2~GeV, while beyond that BEPC-II can run
by adjusting the beam energy. This allows BES-III to study the full range of
vector mesons between the $\pi^+\pi^-$ threshold and 4.6~GeV, which is
the highest energy BEPC-II can reach, thus covering the $\rho$,
$\omega$ and $\phi$, as well as the $\psi$ sector. One will have the chance
to study the excited $\rho$, $\omega$ and $\phi$ states between 1
and about 2.5~GeV. The final states include $\pi^+\pi^-\pi^0$,
$K\bar K$, 4 pions, $\pi\pi K K$, etc. Final states with more than
four particles will be hard to study using the ISR method, since the
$D \bar D$ decay will contribute as background.
