  The well-known relaxed theoretical minimum emittance (TME) cell is commonly
used in the design of multi-bend achromat (MBA) lattices for the new generation
of diffraction limited storage rings. But significantly lower emittance at
moderate focusing properties can be achieved by combining longitudinal gradient
bends (LGB) and reverse bends (RB) in a periodic lattice unit cell. LGBs alone,
however, are of rather limited gain.
  We investigate the emittance achievable for different unit cell classes as a
function of the cell phase advance in a most general framework, i.e. with a
minimum of assumptions on the particular cell optics. Each case is illustrated
with a practical example of a realistic lattice cell, eventually leading to the
LGB/RB unit cell of the baseline lattice for the upgrade of the Swiss Light
Source.
