singles. 

2. The host binaries separation are likely to be be- tween 20 AU to 200 AU for binaries hosting cir- cumstellar planets, and up to a few AU for binaries hosting circumbinary planets. 

3. Planets in post-MS binaries could reside in orbital phase space regions inaccessible to pre-existing first generation planets in such systems (section 3.1). Observations of planets in such orbits could serve as a smoking-gun signature of their second generation origin. 

4. Second generation planets could form even around metal poor stars and/or in metal poor environ- ments such as globular clusters; planetary systems in such environment are likely to be part of evolved binary systems. 

5. Planets showing age inconsistency with (i.e. being younger than) their parent host star could be possi- ble second generation planets candidates. In such systems one may therefore search for a compact object companion to the host star. 

6. Stars in WD binaries showing evidence for mass accretion from a companion star (e.g. chemically peculiar stars such as Barium and CH stars) could be more prone to have second generation planetary companions (for adequate binary separations) 2 

7. Second generation planets could be more massive than regular exo-planets. This may be suggestive that old planet hosting stars with extremely mas- sive planets (or brown dwarf companions found in the brown dwarf dessert) are more likely to be sec- ond generation planetary systems. Again this could point to the existence of a compact object (most likely WD) companion at the appropriate separa- tions. 

8. Planets around compact objects could be second generation planets from mass transfer accretion. These systems are therefore more likely to have a compact object companion. Also, polluted WDs, thought to reflect accretion of asteroids, might be more likely to have WD companions. 

9. The spin-orbit alignment between second genera- tion planets and their host stars is likely to differ from that of first generation planets, and second generation planets might be more likely to show high relative inclinations between different planets in multiple planetary system. 

10. Statistical correlation between planetary compan- ions and their host star metallicity could show a difference between WD binary systems and MS bi- nary systems (but this may be a weak signature, 

2 Note that material from second generation disks and/or plan- ets which was processed in the AGB star would not pollute the star with $^{6}$Li as would first generation planetary material [e.g. see Gonzalez (2006) for a review]. It is not clear to us, however, whether such differences could be detected, and what are the re- quired statistics. 

given that the host star itself accretes metal rich material). Chemically peculiar stars in WD bina- ries may serve as good second generation planetary hosts candidates. 

11. The MS stars in WD-MS binaries may also show higher rotational velocities, due to planetary infall, although such phenomena could also be induced by the re-accretion of matter from the second genera- tion disk. In either case, stars with high rotational velocities in WD binaries could be better candidate hosts of second generation planets. 

## 7. CANDIDATE SECOND GENERATION PLANETARY SYSTEMS 

Observationally, many post-AGB binary stars show evidence for having disks surrounding them either in circumstellar or circumbinary disks (Van Winckel 2004; de Ruyter et al. 2006; Hinkle et al. 2007; van Winckel et al. 2009). Such disks have many similarities with protoplanetary disks as observed in T-Tauri stars, suggesting that new generations of planets could form there(Zuckerman et al. 2008; Melis et al. 2009). 

As discussed above, there are several observational expectations for second generation planetary systems. Currently, several observed planetary systems and plan- etary candidates may be consistent with such expec- tations, and may therefore be considered as candidate second generation planets. These systems are found in evolved binary systems, and include both circum- stellar planets such as in GL 86 (Queloz et al. 2000; Lagrange et al. 2006; Mugrauer & Neuhäuser 2005) and HD 27442 (Mugrauer et al. 2007) as well as several circumbinary planets and candidates: PSR B1620−26 (Backer et al. 1993), HW Vir (Lee et al. 2009), NN Ser (Qian et al. 2009) and DP Leo (Qian et al. 2010b). These systems are consistent with being second gener- ation planetary systems, as their host binary separa- tion is the region where second generation disks could be formed. 

Interestingly, according to Lagrange et al. (2006) the planetary orbit of GL 86 could have been in the forbid- den region for first generation planets (i.e. where they could have not formed and survived) for the most plau- sible parameters for the WD progenitor in this system, which are consistent with its cooling age (Lagrange et al. 2006; Desidera & Barbieri 2007). Although this discrep- ancy could possibly be circumvented by taking different age and mass estimates for the progenitor, under some assumptions (Desidera & Barbieri 2007), a second gener- ation origin for this planet presents an alternative simple and natural explanation. Besides its consistency with being a second generation planet, Gl 86 may therefore show the smoking gun signature of a second generation planetary system. 

Similarly the circumbinary planets candidates in HW Vir, NN Ser and DP Leo are found to be in regions likely to be inaccessible or less favorable for first gen- eration planets in the progenitor pre-evolved binary sys- tems. These planets orbit their evolved binary host at separations of only a few AU, where the pre-stellar evolu- tion orbits of these binaries might have as wide as a few AU (e.g. Ritter 2008), i.e. these planets could not have 

