Search

The orbital period of a cataclysmic variable stands as a crucial parameter for investigating the structure and physics of these binary systems, as well as understanding their evolution. We use photometric Gaia data for dwarf novae (DNe) in the quiescent state which are available for a number of years to determine new orbital periods and improve/modify previously suggested values. Two approaches are implemented for selecting high-inclination targets, either eclipsing or with ellipsoidal variations. We determine new orbital periods for 75 DNe and improve ephemerides for 27 more (three of which change significantly), contributing 9.4% of the known DNe periods of 0.05-2.0 days, and doubling the number of known periods exceeding 0.44 days. Their phase-folded light curves are presented and arranged by orbital period, illustrating the transition from short-period systems, dominated by radiation from the accretion disc and the hot spot, to longer-period DNe, where the Roche-lobe-filling secondary star is the primary visual flux source. This transition -- which occurs around the well-known period gap (around 2-3 hours) -- is expected, as DNe with larger orbital periods typically harbour more massive donors, which contribute to the visible flux. However, this transition is not abrupt. Within the same range of periods, we observe systems dominated by ellipsoidal variations, where the companion star is clearly visible, as well as others dominated by the disc and hot spot. The presence of some DNe with ellipsoidal variations near the lower edge of the period gap is striking, as the companions in these systems are expected to be cool low-mass M-dwarfs not visible in the light curve. This could indicate that we are observing systems where the donor star was originally much more massive and underwent significant nuclear evolution before mass-transfer began, as has been suggested previously for QZ Ser., Comment: Accepted for publication in A&A, 15 pages, 3 figures

Approximately 150 low-mass white dwarfs, with masses below 0.4Msun, have been discovered. The majority of these low-mass WDs are observed in binary systems as they cannot be formed through single-star evolution within the Hubble time. In this study, we present a comprehensive analysis of the double low-mass WD eclipsing binary system J2102-4145. Our investigation involved an extensive observational campaign, resulting in the acquisition of approximately 28 hours of high-speed photometric data across multiple nights using NTT/ULTRACAM, SOAR/Goodman, and SMARTS-1m telescopes. These observations have provided critical insights into the orbital characteristics of this system, including parameters such as inclination and orbital period. To disentangle the binary components of J2102-4145, we employed the XT GRID spectral fitting method with GMOS/Gemini-South and X-Shooter data. Additionally, we used the PHOEBE package for light curve analysis on NTT/ULTRACAM high-speed time-series photometry data to constrain the binary star properties. Our analysis reveals remarkable similarities between the two components of this binary system. For the primary star, we determined Teff1 = 13688 +- 65 K, log g1 = 7.36 +- 0.01, R1 = 0.0211 +- 0.0002 Rsun, and M1 = 0.375 +- 0.003 Msun, while the secondary star is characterized by Teff2 = 12952 +- 53 K, log g2 = 7.32 +- 0.01, R2 = 0.0203 +- 0.0002 Rsun, and M2 = 0.31 +- 0.003 Msun. Furthermore, we observe a notable discrepancy between Teff and R of the less massive WD compared to evolutionary sequences for WDs from the literature, which has significant implications for our understanding of WD evolution. We discuss a potential formation scenario for this system that might explain this discrepancy and explore its future evolution. We predict that this system will merge in about 800 Myr, evolving into a helium-rich hot subdwarf star and later into a hybrid He/CO WD.

The vast majority of close binaries containing a compact object form through common-envelope (CE) evolution. Despite this importance, we struggle to even understand the energy budget of CE evolution. For decades, observed long-period post-CE binaries have been interpreted as evidence for additional energies to contribute during CE evolution. We have recently shown that this argument is based on simplified assumptions for all long-period post-CE binaries containing massive white dwarfs. The only remaining post-CE binary star that has been claimed to require contributions from additional energy sources to understand its formation is KOI 3278. Here we address in detail the potential evolutionary history of KOI 3278. In particular, we investigated whether extra energy sources, such as recombination energy, are indeed required to explain its existence. We used the 1D stellar evolution code MESA to carry out binary evolution simulations and searched for potential formation pathways for KOI 3278 that are able to explain its observed properties. We found that KOI 3278 can be explained if the white dwarf progenitor filled its Roche lobe during a helium shell flash. In this case, the orbital period of KOI 3278 can be reproduced if the CE binding energy is calculated taking into account gravitational energy and thermodynamic internal energy. While the CE evolution that led to the formation of KOI 3278 must have been efficient, that is, most of the available orbital energy must have been used to unbind the CE, recombination energy is not required. We conclude that currently not a single observed post-CE binary requires to assume energy sources other than gravitational and thermodynamic energy to contribute to CE evolution. KOI 3278, however, remains an intriguing post-CE binary as, unlike its siblings, understanding its existence requires highly efficient CE ejection., Comment: Accepted for publication in A&A

In this first in a series of papers related to long-period post-common-envelope (CE) binaries, we investigated whether extra energy is required or not to explain the currently known post-CE binaries with sufficiently long orbital periods consisting of oxygen-neon white dwarfs with AFGK-type main-sequence star companions. We carried out binary population simulations with the BSE code and searched for their formation pathways. Unlike what has been claimed for a long time, we show that all such post-CE binaries can be explained by assuming inefficient CE evolution, which is consistent with results achieved for the remaining post-CE binaries. There is therefore no need for an extra energy source. We also found that for CE efficiency close to 100%, post-CE binaries hosting oxygen-neon white dwarfs with orbital periods as long as a thousand days can be explained. For all known systems we found formation pathways consisting of CE evolution triggered when a highly evolved (i.e. the envelope mass being comparable to the core mass) thermally-pulsing asymptotic giant branch star fills its Roche lobe at an orbital period of several thousand days. Due to the sufficiently low envelope mass and sufficiently long orbital period, the resulting post-CE orbital period can easily be several tens of days. We conclude that the known post-CE binaries with oxygen-neon white dwarfs and AFGK-type main-sequence stars can be explained without invoking any energy source other than orbital and thermal energy. Our results strengthen the idea that the most common formation pathway of the overall population of post-CE binaries hosting white dwarfs is through inefficient CE evolution., Comment: Accepted for publication in A&A

Hot subdwarf B (sdB) stars are helium core burning stars that have lost almost their entire hydrogen envelope due to binary interaction. Their assumed canonical mass of $\rm M_{\mathrm{sdB}}\sim0.47 M_{\odot}$ has recently been debated given a broad range found both from observations as well as from the simulations. Here, we revise and refine the mass range for sdBs derived two decades ago with the Eggleton code, using the stellar evolution code MESA, and discuss the effects of metallicity and the inclusion of core overshooting during the main sequence. We find an excellent agreement for low-mass progenitors, up to $\sim2.0 \rm M_{\odot}$. For stars more massive than $\sim2.5 \rm M_{\odot}$ we obtain a wider range of sdB masses compared to the simulations from the literature. Our MESA models for the lower metallicity predict, on average, slightly more massive sdBs. Finally, we show the results for the sdB lifetime as a function of sdB mass and discuss the effect this might have in the comparison between simulations and observational samples. This study paves the way for reproducing the observed Galactic mass distribution of sdB binaries., Comment: Accepted for publication in MNRAS

Some white dwarfs accreting from non-degenerate companions show anomalous carbon and nitrogen abundances in the photospheres of their stellar components which have been postulated to be descendants of supersoft X-ray binaries. Therefore the carbon-to-nitrogen ratio can provide constraints upon their past evolution. We fit far ultraviolet spectroscopy of the cataclysmic variable HS0218+3229 taken with the Cosmic Origins Spectrograph using Markov Chain Monte Carlo. While some parameters depend upon the amount of reddening, the carbon-to-nitrogen ratio is about one tenth of the Solar value ($log C/N=-0.53^{+0.13}_{-0.14}$ and $-0.58^{+0.16}_{-0.15}$ for almost no reddening and E(B-V)=0.065, respectively, which are consistent within the uncertainties). We also provide estimates of the silicon and aluminum abundances, and upper limits for iron and oxygen. Using the measured parameters of HS0218+3229 we reconstruct its past using evolutionary simulations with MESA. We implemented Gaussian process fits to the MESA grid in order to determiner the most likely initial binary configuration of HS0218+3229. We found that an initial mass of the donor of $M_{\rm donor;i}=0.90-0.98,\mathrm{M}_{\odot}$ and an initial orbital period of $P_{\rm orb;i}=2.88$ days ($P_{\rm orb;i}=3.12-3.16$ days) for an assumed white dwarf mass of $M_{\mathrm{WD}}=0.83\,\mathrm{M}_{\odot}$ ($M_{\mathrm{WD}}=0.60\,\mathrm{M}_{\odot}$) are needed to replicate the measured parameters. These configurations imply that the system did not go through a phase of quasi-steady hydrogen-burning on the white dwarf's surface. However, it could have experienced a phase of thermal timescale mass transfer in the past if the initial mass ratio was $\geq1.5$. We predict that HS0218+3229 will evolve into a CV with a period below the $\simeq80$\,min period minimum for normal CVs, displaying helium and hydrogen in its spectrum.

Common envelope evolution is a fundamental ingredient in our understanding of the formation of close binary stars containing compact objects which includes the progenitors of type Ia supernovae, short gamma ray bursts and most stellar gravitational wave sources. To predict the outcome of common envelope evolution we still rely to a large degree on a simplified energy conservation equation. Unfortunately, this equation contains a theoretically rather poorly constrained efficiency parameter ($\alpha_{\mathrm{CE}}$) and, even worse, it is unclear if energy sources in addition to orbital energy (such as recombination energy) contribute to the envelope ejection process. In previous works we reconstructed the evolution of observed populations of post common envelope binaries (PCEBs) consisting of white dwarfs with main sequence star companions and found indications that the efficiency is rather small ($\alpha_{\mathrm{CE}}\simeq0.2-0.3$) and that extra energy sources are only required in very few cases. Here we used the same reconstruction tool to investigate the evolutionary history of a sample of observed PCEBs with brown dwarf companions. In contrast to previous works, we found that the evolution of observationally well characterized PCEBs with brown dwarf companions can be understood assuming a low common envelope efficiency ($\alpha_{\mathrm{CE}}=0.24-0.41$), similar to that required to understand PCEBs with main sequence star companions, and that contributions from recombination energy are not required. We conclude that the vast majority of PCEBs form from common envelope evolution that can be parameterized with a small efficiency and without taking into account additional energy sources., Comment: Accepted by MNRAS. 10 pages, 1 figure, 2 tables

The origin of strong magnetic fields in white dwarfs has been a puzzle for decades. Recently, a dynamo mechanism operating in rapidly rotating and crystallizing white dwarfs has been suggested to explain the occurrence rates of strong magnetic fields in white dwarfs with close low-mass main sequence star companions. Here we investigate whether the same mechanism may produce strong magnetic fields in close double white dwarfs. The only known strongly magnetic white dwarf that is part of a close double white dwarf system, the magnetic component of NLTT 12758, is rapidly rotating and likely crystallizing and therefore the proposed dynamo mechanism represents an excellent scenario for the origin of its magnetic field. Presenting a revised formation scenario for NLTT 12758, we find a natural explanation for the rapid rotation of the magnetic component. We furthermore show that it is not surprising that strong magnetic fields have not been detected in all other known double white dwarfs. We therefore conclude that the incidence of magnetic fields in close double white dwarfs supports the idea that a rotation and crystallization driven dynamo plays a major role in the generation of strong magnetic fields in white dwarfs., Comment: accepted for publication in MNRAS

We present an analysis of mock X-ray spectra and light curves of magnetic cataclysmic variables using an upgraded version of the 3D CYCLOPS code. This 3D representation of the accretion flow allows us to properly model total and partial occultation of the post-shock region by the white dwarf as well as the modulation of the X-ray light curves due to the phase-dependent extinction of the pre-shock region. We carried out detailed post-shock region modeling in a four-dimensional parameter space by varying the white dwarf mass and magnetic field strength as well as the magnetosphere radius and the specific accretion rate. To calculate the post-shock region temperature and density profiles, we assumed equipartition between ions and electrons, took into account the white dwarf gravitational potential, the finite size of the magnetosphere and a dipole-like magnetic field geometry, and considered cooling by both bremsstrahlung and cyclotron radiative processes. By investigating the impact of the parameters on the resulting X-ray continuum spectra, we show that there is an inevitable degeneracy in the four-dimensional parameter space investigated here, which compromises X-ray continuum spectral fitting strategies and can lead to incorrect parameter estimates. However, the inclusion of X-ray light curves in different energy ranges can break this degeneracy, and it therefore remains, in principle, possible to use X-ray data to derive fundamental parameters of magnetic cataclysmic variables, which represents an essential step toward understanding their formation and evolution., Comment: 42 pages, accepted for publication in The Astrophysical Journal Supplement Series

Recently, Bahramian et al. investigated a large sample of globular clusters (GCs) and found that bright intermediate polars (IPs) are a factor of 10 less frequent in GCs than in the Galactic field. We theoretically investigate here this discrepancy based on GC numerical simulations. We found that, due to disruptive dynamical interaction, there is on average a reduction of only half of bright IP progenitors, which is clearly not enough to explain the observed deficiency. However, if the rotation- and crystallization-driven dynamo scenario recently proposed by Schreiber et al. is incorporated in the simulations, the observed rareness of bright IPs in GCs can be reproduced. This is because bright cataclysmic variables in GCs are typically very old systems ($\gtrsim$ 10 Gyr), with white dwarfs that almost fully crystallized before mass transfer started, which does not allow strong magnetic fields to be generated. The observed mass density of bright IPs in GCs can be recovered if around one third of the bright cataclysmic variables dynamically formed through mergers have magnetic field strengths similar to those of IPs. We conclude that the observed paucity of bright IPs in GCs is a natural consequence of the newly proposed rotation- and crystallization-driven dynamo scenario., Comment: letter accepted for publication by MNRAS

The origin of magnetic fields in white dwarfs remains a fundamental unresolved problem in stellar astrophysics. In particular, the very different fractions of strongly (exceeding 1 MG) magnetic white dwarfs in evolutionarily linked populations of close white dwarf binary stars cannot be reproduced by any scenario suggested so far. Strongly magnetic white dwarfs are absent among detached white dwarf binary stars that are younger than approximately 1 Gyr. In contrast, in semi-detached cataclysmic variables in which the white dwarf accretes from a low-mass star companion, more than one third host a strongly magnetic white dwarf. Here we present binary star evolutionary models that include the spin evolution of accreting white dwarfs and crystallization of their cores, as well as magnetic field interactions between both stars. We show that a crystallization- and rotation-driven dynamo similar to those working in planets and low-mass stars can generate strong magnetic fields in the white dwarfs in cataclysmic variables which explains their large fraction among the observed population. When the magnetic field generated in the white dwarfs connects with that of the secondary stars, synchronization torques and reduced angular momentum loss cause the binary to detach for a relatively short period of time. The few known strongly magnetic white dwarfs in detached binaries, including AR Sco, are in this detached phase.

We present the first population synthesis of synchronous magnetic cataclysmic variables, called polars, taking into account the effect of the white dwarf (WD) magnetic field on angular momentum loss. We implemented the reduced magnetic braking (MB) model proposed by Li, Wu & Wickramasinghe into the Binary Stellar Evolution (BSE) code recently calibrated for cataclysmic variable (CV) evolution. We then compared separately our predictions for polars and non-magnetic CVs with a large and homogeneous sample of observed CVs from the Sloan Digital Sky Survey. We found that the predicted orbital period distributions and space densities agree with the observations if period bouncers are excluded. For polars, we also find agreement between predicted and observed mass transfer rates, while the mass transfer rates of non-magnetic CVs with periods ${\gtrsim3}$ hr drastically disagree with those derived from observations. Our results provide strong evidence that the reduced MB model for the evolution of highly magnetized accreting WDs can explain the observed properties of polars. The remaining main issues in our understanding of CV evolution are the origin of the large number of highly magnetic WDs, the large scatter of the observed mass transfer rates for non-magnetic systems with periods ${\gtrsim3}$ hr, and the absence of period bouncers in observed samples., Comment: 15 pages, accepted for publication in MNRAS

Although the theory of cataclysmic variable (CV) evolution is able to explain several observational aspects, strong discrepancies have existed for decades between observations and theoretical predictions of the orbital period distribution, the location of the minimum period, and the space density of CVs. Moreover, it has been shown in the last decade that the average white dwarf (WD) mass observed in CVs is significantly higher than the average mass in single WDs or in detached progenitors of CVs, and that there is an absence of helium-core WDs in CVs which is not observed in their immediate detached progenitors. This highly motivated us to revise the theory of CV formation and evolution. A new empirical model for angular momentum loss in CVs was developed in order to explain the high average WD mass observed and the absence of systems with helium-core WDs. This model seems to help, at the same time, with all of the above mentioned disagreements between theory and observations. Moreover, it also provides us with a very likely explanation for the existence of low-mass WDs without a companion. Here we will review the standard model for CV evolution and the disagreements that have existed for decades between simulations and observations with their possible solutions and/or improvements. We will also summarize the recently confirmed disagreement related to the average WD mass and the fraction of helium-core WDs among CVs, as well as the development of an empirical model that allows us to solve all the disagreements, discussing the physics that could be involved., Comment: Accepted for publication in Advances in Space Research

Interacting binaries containing white dwarfs can lead to a variety of outcomes that range from powerful thermonuclear explosions, which are important in the chemical evolution of galaxies and as cosmological distance estimators, to strong sources of low frequency gravitational wave radiation, which makes them ideal calibrators for the gravitational low-frequency wave detector LISA mission. However, current theoretical evolution models still fail to explain the observed properties of the known populations of white dwarfs in both interacting and detached binaries. Major limitations are that the existing population models have generally been developed to explain the properties of sub-samples of these systems, occupying small volumes of the vast parameter space, and that the observed samples are severely biased. The overarching goal for the next decade is to assemble a large and homogeneous sample of white dwarf binaries that spans the entire range of evolutionary states, to obtain precise measurements of their physical properties, and to further develop the theory to satisfactorily reproduce the properties of the entire population. While ongoing and future all-sky high- and low-resolution optical spectroscopic surveys allow us to enlarge the sample of these systems, high-resolution ultraviolet spectroscopy is absolutely essential for the characterization of the white dwarfs in these binaries. The Hubble Space Telescope is currently the only facility that provides ultraviolet spectroscopy, and with its foreseeable demise, planning the next ultraviolet mission is of utmost urgency., Comment: 5 pages, 2 figures Science White Paper submitted to Astro2020 Decadal Survey

The predicted and observed space density of cataclysmic variables (CVs) have been for a long time discrepant by at least an order of magnitude. The standard model of CV evolution predicts that the vast majority of CVs should be period bouncers, whose space density has been recently measured to be $\rho \lesssim 2 \times 10^{-5}$ pc$^{-3}$. We performed population synthesis of CVs using an updated version of the Binary Stellar Evolution (BSE) code for single and binary star evolution. We find that the recently suggested empirical prescription of consequential angular momentum loss (CAML) brings into agreement predicted and observed space densities of CVs and period bouncers. To progress with our understanding of CV evolution it is crucial to understand the physical mechanism behind empirical CAML. Our changes to the BSE code are also provided in details, which will allow the community to accurately model mass transfer in interacting binaries in which degenerate objects accrete from low-mass main-sequence donor stars., Comment: 11 pages, 4 figures, 1 table; accepted for publication in MNRAS

We report the discovery of HE 0430-2457, the first extremely low-mass pre-white dwarf (ELM pre-WD) in a long period binary (P = 771 +- 3 d). The spectroscopic parameters of the primary are determined to be Teff = 26200 +- 1500 K and logg = 5.40 +- 0.35, placing it in the region occupied by core He-burning hot subdwarf B stars. By comparing the spectroscopic parameters of the K-type companion to stellar models, and using the mass ratio, the mass of the hot primary is determined to be 0.23 Msun. Given that this is too low for core He-burning, the primary in HE 0430-2457 is not an EHB star but a pre WD of the ELM type. As the lifetime of ELM pre-WDs in this region of the HR diagram populated by EHBs is thought to be very short, they are not considered to be part of the observed EHBs. However, the discovery of this system indicates that the percentage of ELM pre-WDs in the observed EHB population might be higher than previously thought. Binary evolution models indicate that HE 0430-2457 is likely formed by a merger of the inner binary in a hierarchical triple system., Comment: 5 pages, 3 figures, Accepted for publication in MNRAS

In this third of a series of papers related to cataclysmic variables (CVs) and related objects, we analyse the population of CVs in a set of 12 globular cluster models evolved with the MOCCA Monte Carlo code, for two initial binary populations (IBPs), two choices of common-envelope phase (CEP) parameters, and three different models for the evolution of CVs and the treatment of angular momentum loss. When more realistic models and parameters are considered, we find that present-day cluster CV duty cycles are extremely-low ($\lesssim 0.1$ per cent) which makes their detection during outbursts rather difficult. Additionally, the IBP plays a significant role in shaping the CV population properties, and models that follow the Kroupa IBP are less affected by enhanced angular momentum loss. We also predict from our simulations that CVs formed dynamically in the past few Gyr (massive CVs) correspond to bright CVs (as expected), and that faint CVs formed several Gyr ago (dynamically or not) represent the overwhelming majority. Regarding the CV formation rate, we rule out the notion that it is similar irrespective of the cluster properties. Finally, we discuss the differences in the present-day CV properties related to the IBPs, the initial cluster conditions, the CEP parameters, formation channels, the CV evolution models, and the angular momentum loss treatments., Comment: 18 pages, 7 figures, 2 tables; accepted for publication in MNRAS

Low-mass helium-core white-dwarfs (WDs) with masses below 0.5 Msun are known to be formed in binary star systems but unexpectedly a significant fraction of them seem to be single. On the other hand, in Cataclysmic Variables (CVs) a large number of low-mass WD primary stars is predicted but not observed. We recently showed that the latter problem can be solved if consequential angular momentum loss causes especially CVs with low-mass WDs to merge and form single stars. Here we simulate the population of single WDs resulting from single star evolution and from binary star mergers taking into account these new merging CVs. We show that according to the revised model of CV evolution, merging CVs might be the dominant channel leading to the formation of low-mass single WDs and that the predicted relative numbers are consistent with observations. This can be interpreted as further evidence for the revised model of CV evolution we recently suggested. This model includes consequential angular momentum loss that increases with decreasing WD mass and might not only explain the absence of low-mass WD primaries in CVs but also the existence of single low-mass WDs., Comment: 5 pages, 3 figures. Accepted for publication in MNRAS

A central hypothesis in the theory of cataclysmic variable (CV) evolution is the need to explain the observed lack of accreting systems in the ~2-3 h orbital period range, known as the period gap. The standard model, disrupted magnetic braking (DMB), reproduces the gap by postulating that CVs transform into inconspicuous detached white dwarf (WD) plus main sequence (MS) systems, which no longer resemble CVs. However, observational evidence for this standard model is currently indirect and thus this scenario has attracted some criticism throughout the last decades. Here we perform a simple but exceptionally strong test of the existence of detached CVs (dCVs). If the theory is correct dCVs should produce a peak in the orbital period distribution of detached close binaries consisting of a WD and an M4-M6 secondary star. We measured six new periods which brings the sample of such binaries with known periods below 10 h to 52 systems. An increase of systems in the ~2-3 h orbital period range is observed. Comparing this result with binary population models we find that the observed peak can not be reproduced by PCEBs alone and that the existence of dCVs is needed to reproduce the observations. Also, the WD mass distribution in the gap shows evidence of two populations in this period range, i.e. PCEBs and more massive dCVs, which is not observed at longer periods. We therefore conclude that CVs are indeed crossing the gap as detached systems, which provides strong support for the DMB theory., Comment: 12 pages, 6 figures, 2 tables, accepted for publication in MNRAS

The observed orbital period distribution of cataclysmic variables (CVs), the space density derived from observations, and the observed orbital period minimum are known to disagree with theoretical predictions since decades. More recently, the white dwarf (WD) masses in CVs have been found to significantly exceed those of single WDs, which is in contrast to theoretical expectations as well. We here claim that all these problems are related and can be solved if CVs with low-mass white dwarfs are driven into dynamically unstable mass transfer due to consequential angular momentum loss (CAML). Indeed, assuming CAML increases as a function of decreasing white dwarf mass can bring into agreement the predictions of binary population models and the observed properties of the CV population. We speculate that a common envelope like evolution of CVs with low-mass WDs following a nova eruption might be the physical process behind our empirical prescription of CAML., Comment: To appear in the proceedings of the conference "The Golden Age of Cataclysmic Variables and Related Objects - III" held in Palermo, Italy, 7-12 September 2015

The average white dwarf (WD) masses in cataclysmic variables (CVs) have been measured to significantly exceed those of single WDs, which is the opposite of what is theoretically expected. We present the results of binary population synthesis models taking into account consequential angular momentum loss (CAML) that is assumed to increase with decreasing WD mass. This approach can not only solve the WD mass problem, but also brings in agreement theoretical predictions and observations of the orbital period distribution and the space density of CVs. We speculate that frictional angular momentum loss following nova eruptions might cause such CAML and could thus be the missing ingredient of CV evolution., Comment: accepted for publication in MNRAS Letters