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{\bf Abstract Registration Number 571} \\
{\bf Parallel sessions: 5,4} \\
{\bf Plenary sessions: 7,6}
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\abstractheader{\bf Measurements and Interpretation of the Diffractive 
Structure \\ Function $F_2^{D(3)}$ at HERA\\
H1 Collaboration, DESY}

\abstractbody{
New measurements of the contribution $F_2^{D(3)}$ to the proton structure
function $F_2$ from interactions of the type $ep\rightarrow eXY$ 
are presented which extend
the kinematic range to both higher and lower $Q^2$ than hitherto
published. A dominant diffractive exchange together with additional
subleading reggeon contributions reproduce well the 
$x_{_{\rm I\!P}}$ dependence
of $F_2^{D(3)}$.  In a fit to the new low $Q^2$ data 
together with the previous
published measurements, the diffractive trajectory is found to
have an intercept which is consistent with that already published. A
fit is also performed using a QCD-inspired
model in which parton distributions are assigned to the leading and
sub-leading exchanges. When the new low $Q^2$ data are included, the 
results are found to be consistent with
those already published. 
The contribution to $F_2^{D(3)}$ from higher
twist processes is found to be important at low $Q^2$ and large $\beta$.
The ratio, $r_D^{(3)}$, of the
diffractive cross section, for different fixed values of $\beta$, to
the total deep-inelastic cross section is evaluated as a function of
Bjorken-x and $Q^2$. The results are discussed in terms of the
properties of the diffractive exchange in the deep-inelastic domain.
}

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