The intensities of lines of the components found by solution of Eq. (6 (click here)) are expressed in ratio to this common continuum. If in another exposure the spectrum of component "1'' is increased (or decreased if

Specially, in our example shown in Fig. 1 (click here), both components have central line intensities 50% of their individual continua. In the maximum (left panel) the primary is twice as bright as the secondary, hence the lines of primary and secondary have depths of and of the total observed continuum, respectively. Let in the primary eclipse (right panel) the intensity of the primary be decreased to -times its normal value. The absolute depth of the line of primary is then 4-times lower while that of the secondary is unchanged. However, in ratio to the total continuum, which is now one half of its maximum value, the depth of primary line is only twice lower, while the intensity of the secondary line is twice higher.

Solving Eqs. (9 (click here)) and (10 (click here)), the factor *z* can be
simply expressed as

and the ratio of continua intensities

Obviously, if *z*<1, then and . This behavior
can help to distinguish the variations caused by the
"geometrical'' reasons (or their equivalent) from intrinsic
variations of line intensities of a component or from the
observational noise.

In the usual case of *k* exposures, the factors
of the darkenings of the component "1'' can be calculated independently
for each exposure from according to Eq. (11 (click here)).
The ratio of continua intensities can be then obtained by least
square fit of Eqs. (9 (click here)) and (10 (click here)) e.g. on a logarithmic
(i.e. magnitude) scale, i.e. by solving the condition

for the variation , i.e. simply by numerical
minimization of function . The differences of magnitudes
of both components at the time of chosen exposure can be then
expressed by

If the secondary component is supposed to be constant during
the primary eclipse, then the magnitude of the whole system
is given by

Vice versa, the total magnitude of the system in the course of
secondary eclipse can be estimated by

The applicability of the method of relative line photometry
in the above given approximation is certainly limited by the
implicitly included assumptions that
(i) the change of intensity is the same for the line and
the continuum of the eclipsed component,
(ii) the intensities of both component are constant outside
their eclipses and
(iii) that the shape of line profile is constant.
The first assumption can be violated even in the case of a pure
eclipse due to the limb darkening, which is different in the
continuum and in the line. This problem will be studied in the next
section.
The second assumption is crucial to have well defined the "normal''
state (in which are directly observed), so that the
line-intensity of the non-eclipsed component can be used as
a "comparison''. This assumption can be violated e.g. for
ellipsoidal variables. Obviously, a more sophisticated procedure
(analogous to the light-curves solution) of fitting the observed
values of (related to a reasonably chosen "reference''
state of ) by a theoretical model can yield an
additional information in such a case.
Finally, the third assumption can be violated e.g. by Rossiter
effect when different equatorial parts of a rotating star are
eclipsed. This assumption is crucial for the disentangling
according to Eq. (1 (click here)). However, as far as the rotational
broadening (even for a partially eclipsed component) is also
given by a convolution in *x*, it is, in principle, possible
to generalize the method of the disentangling and line
photometry even to this case.
On the other hand, such
effects can be negligible in many cases not only for a pure
eclipse, but in some approximation also for elliptic, reflecting
or pulsating components of binaries.

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