The element yield 
of a star of initial mass m is defined as
the newly formed and ejected mass of element j integrated over the
stellar lifetime 
and normalized to the initial mass (e.g.
Maeder 1992):
where E(m,t) denotes the stellar mass-loss rate and
the abundance by mass of element j in the ejecta at
stellar age t. Note that negative yields may occur e.g. in the case of
hydrogen consumption.
In general, stellar yields depend on the initial stellar abundances
in different manners. First, abundances in the
stellar envelope 
are related in a complex manner to
the initial abundances of distinct elements (e.g. helium and/or oxygen).
This is particularly true for stars on the AGB as we will discuss below.
To first order, we take this
important effect into account by the dependence of the evolutionary algorithms
used on the initial stellar metallicity Z(0), i.e. integrated over
elements heavier than helium.
Second, stellar lifetimes 
, remnant
masses 
, and mass-loss rates E(m,t) vary
strongly with initial stellar metallicity (e.g. Schaller et al. 1992).
Third, stellar yields are defined with respect to the initial stellar
abundances 
(cf. Eq. 1).
To allow for a direct comparison of
the derived element yields for pre-AGB and AGB evolution phases (see below),
we adopt the initial abundances as used in the stellar evolution tracks
presented by the Geneva group (i.e. Schaller et al. 1992;
Schaerer et al.
1993a,b; Charbonnel et al. 1993;
Meynet et al. 1994).
In brief, the Geneva group calculated the initial helium abundance from:
assuming a primordial helium abundance 
of 0.24 (e.g. Audouze
1987; Steigman 1989) and 
= 3 (e.g.\
Pagel et al. 1986; Pagel & Kaztauskas 1992)
for stars in the Galactic disk.
Accordingly,
these tracks imply a revised solar metallicity of 
with
(see Schaller et al. 1992). Initial
abundances of C, N, and O were taken according to the relative
ratios (cf. Anders & Grevesse 1989) used in the opacity tables by
Rogers & Iglesias (1992). The hydrogen content was calculated from
X= 1 - Y - Z.
Table 1 (click here) lists the adopted initial abundances of H, 
He, 
C,
C, 
N, and 
O at metallicities
Z(0) = 0.001, 0.004, 0.008, 0.02, and 0.04. Note that abundances are
given by mass throughout this paper.
Table 1: Initial element abundances adopted
In this paper, we distinguish stellar yields e.g. for the pre-AGB and AGB
phases (cf. Sect. 4). In this case, the total mass of
element j ejected during mass-loss phase i (with age boundaries
and 
in Eq. 1) can be written as:
where 
is the
total mass ejected during phase i. Similarly, mean abundances of
element j within the ejecta returned to the ISM during phase i can be
written as:
The lifetime-integrated stellar yield of element j in terms of the
stellar yields for distinct mass-loss phases i is given by
. Since
= 0 and 
= 1
according to Eq. (1), the total stellar mass ejected can be expressed as:
where 
is the stellar remnant
mass. In this manner, Eq. (1) also can be written as:
