  Algol-type binary systems are the product of rapid mass transfer between the
initially more massive component to its companion. It is still unknown whether
the process is conservative, or whether substantial mass is lost from the
system. The history of a system prior to mass exchange is imprinted in the
photospheric chemical composition, in particular in the carbon-to-nitrogen
(C/N) ratio. We use this to trace the efficiency of mass-transfer processes in
the components of a classical Algol-type system, $\delta$ Librae. The present
analysis is based on new spectroscopic data (ground-based high-resolution
\'echelle spectra) and extracted archival photometric observations (space-based
measurements from the STEREO satellites). In the orbital solution,
non-Keplerian effects on the radial-velocity variations were taken into
account. This reduces the primary's mass by 1.1 \Msun\ ($\sim$23\%)
significantly in comparison to previous studies, and removes a long-standing
discrepancy between the radius and effective temperature. A spectral
disentangling technique is applied to the \'echelle observations and the
spectra of the individual components are separated. Atmospheric and abundance
analyses are performed for the mass-gaining component and we found C/N $= 1.55
\pm 0.40$ for this star. An extensive set of evolutionary models
($3.5\times10^6$) for both components are calculated from which the
best-fitting model is derived. It is found that $\beta$, the parameter which
quantifies the efficiency of mass-loss from a binary system, is close to zero.
This means that the mass-transfer in $\delta$ Lib is mostly conservative with
little mass loss from the system.
